feat(scanner): implement selective folder scanning and file system watcher improvements (#4674)

* feat: Add selective folder scanning capability

Implement targeted scanning of specific library/folder pairs without
full recursion. This enables efficient rescanning of individual folders
when changes are detected, significantly reducing scan time for large
libraries.

Key changes:
- Add ScanTarget struct and ScanFolders API to Scanner interface
- Implement CLI flag --targets for specifying libraryID:folderPath pairs
- Add FolderRepository.GetByPaths() for batch folder info retrieval
- Create loadSpecificFolders() for non-recursive directory loading
- Scope GC operations to affected libraries only (with TODO for full impl)
- Add comprehensive tests for selective scanning behavior

The selective scan:
- Only processes specified folders (no subdirectory recursion)
- Maintains library isolation
- Runs full maintenance pipeline scoped to affected libraries
- Supports both full and quick scan modes

Examples:
  navidrome scan --targets "1:Music/Rock,1:Music/Jazz"
  navidrome scan --full --targets "2:Classical"

* feat(folder): replace GetByPaths with GetFolderUpdateInfo for improved folder updates retrieval

Signed-off-by: Deluan <deluan@navidrome.org>

* test: update parseTargets test to handle folder names with spaces

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(folder): remove unused LibraryPath struct and update GC logging message

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(folder): enhance external scanner to support target-specific scanning

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): simplify scanner methods

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(watcher): implement folder scanning notifications with deduplication

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(watcher): add resolveFolderPath function for testability

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(watcher): implement path ignoring based on .ndignore patterns

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): implement IgnoreChecker for managing .ndignore patterns

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(ignore_checker): rename scanner to lineScanner for clarity

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): enhance ScanTarget struct with String method for better target representation

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(scanner): validate library ID to prevent negative values

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): simplify GC method by removing library ID parameter

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(scanner): update folder scanning to include all descendants of specified folders

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(subsonic): allow selective scan in the /startScan endpoint

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): update CallScan to handle specific library/folder pairs

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): streamline scanning logic by removing scanAll method

Signed-off-by: Deluan <deluan@navidrome.org>

* test: enhance mockScanner for thread safety and improve test reliability

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): move scanner.ScanTarget to model.ScanTarget

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor: move scanner types to model,implement MockScanner

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): update scanner interface and implementations to use model.Scanner

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(folder_repository): normalize target path handling by using filepath.Clean

Signed-off-by: Deluan <deluan@navidrome.org>

* test(folder_repository): add comprehensive tests for folder retrieval and child exclusion

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): simplify selective scan logic using slice.Filter

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): streamline phase folder and album creation by removing unnecessary library parameter

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): move initialization logic from phase_1 to the scanner itself

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(tests): rename selective scan test file to scanner_selective_test.go

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(configuration): add DevSelectiveWatcher configuration option

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(watcher): enhance .ndignore handling for folder deletions and file changes

Signed-off-by: Deluan <deluan@navidrome.org>

* docs(scanner): comments

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(scanner): enhance walkDirTree to support target folder scanning

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(scanner, watcher): handle errors when pushing ignore patterns for folders

Signed-off-by: Deluan <deluan@navidrome.org>

* Update scanner/phase_1_folders.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* refactor(scanner): replace parseTargets function with direct call to scanner.ParseTargets

Signed-off-by: Deluan <deluan@navidrome.org>

* test(scanner): add tests for ScanBegin and ScanEnd functionality

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(library): update PRAGMA optimize to check table sizes without ANALYZE

Signed-off-by: Deluan <deluan@navidrome.org>

* test(scanner): refactor tests

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(ui): add selective scan options and update translations

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(ui): add quick and full scan options for individual libraries

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(ui): add Scan buttonsto the LibraryList

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(scan): update scanning parameters from 'path' to 'target' for selective scans.

* refactor(scan): move ParseTargets function to model package

* test(scan): suppress unused return value from SetUserLibraries in tests

* feat(gc): enhance garbage collection to support selective library purging

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(scanner): prevent race condition when scanning deleted folders

When the watcher detects changes in a folder that gets deleted before
the scanner runs (due to the 10-second delay), the scanner was
prematurely removing these folders from the tracking map, preventing
them from being marked as missing.

The issue occurred because `newFolderEntry` was calling `popLastUpdate`
before verifying the folder actually exists on the filesystem.

Changes:
- Move fs.Stat check before newFolderEntry creation in loadDir to
  ensure deleted folders remain in lastUpdates for finalize() to handle
- Add early existence check in walkDirTree to skip non-existent target
  folders with a warning log
- Add unit test verifying non-existent folders aren't removed from
  lastUpdates prematurely
- Add integration test for deleted folder scenario with ScanFolders

Fixes the issue where deleting entire folders (e.g., /music/AC_DC)
wouldn't mark tracks as missing when using selective folder scanning.

* refactor(scan): streamline folder entry creation and update handling

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(scan): add '@Recycle' (QNAP) to ignored directories list

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(log): improve thread safety in logging level management

* test(scan): move unit tests for ParseTargets function

Signed-off-by: Deluan <deluan@navidrome.org>

* review

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: deluan <deluan.quintao@mechanical-orchard.com>

model/scanner.go

@@ -0,0 +1,81 @@
+package model
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// ScanTarget represents a specific folder within a library to be scanned.
+// NOTE: This struct is used as a map key, so it should only contain comparable types.
+type ScanTarget struct {
+	LibraryID  int
+	FolderPath string // Relative path within the library, or "" for entire library
+}
+
+func (st ScanTarget) String() string {
+	return fmt.Sprintf("%d:%s", st.LibraryID, st.FolderPath)
+}
+
+// ScannerStatus holds information about the current scan status
+type ScannerStatus struct {
+	Scanning    bool
+	LastScan    time.Time
+	Count       uint32
+	FolderCount uint32
+	LastError   string
+	ScanType    string
+	ElapsedTime time.Duration
+}
+
+type Scanner interface {
+	// ScanAll starts a scan of all libraries. This is a blocking operation.
+	ScanAll(ctx context.Context, fullScan bool) (warnings []string, err error)
+	// ScanFolders scans specific library/folder pairs, recursing into subdirectories.
+	// If targets is nil, it scans all libraries. This is a blocking operation.
+	ScanFolders(ctx context.Context, fullScan bool, targets []ScanTarget) (warnings []string, err error)
+	Status(context.Context) (*ScannerStatus, error)
+}
+
+// ParseTargets parses scan targets strings into ScanTarget structs.
+// Example: []string{"1:Music/Rock", "2:Classical"}
+func ParseTargets(libFolders []string) ([]ScanTarget, error) {
+	targets := make([]ScanTarget, 0, len(libFolders))
+
+	for _, part := range libFolders {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+
+		// Split by the first colon
+		colonIdx := strings.Index(part, ":")
+		if colonIdx == -1 {
+			return nil, fmt.Errorf("invalid target format: %q (expected libraryID:folderPath)", part)
+		}
+
+		libIDStr := part[:colonIdx]
+		folderPath := part[colonIdx+1:]
+
+		libID, err := strconv.Atoi(libIDStr)
+		if err != nil {
+			return nil, fmt.Errorf("invalid library ID %q: %w", libIDStr, err)
+		}
+		if libID <= 0 {
+			return nil, fmt.Errorf("invalid library ID %q", libIDStr)
+		}
+
+		targets = append(targets, ScanTarget{
+			LibraryID:  libID,
+			FolderPath: folderPath,
+		})
+	}
+
+	if len(targets) == 0 {
+		return nil, fmt.Errorf("no valid targets found")
+	}
+
+	return targets, nil
+}