feat(server): add percentage-based limits to smart playlists (#5144)

* feat(playlists): add percentage-based limits to smart playlists

Add a new `limitPercent` JSON field to Criteria that allows smart playlist
limits to be expressed as a percentage of matching tracks rather than a
fixed number. For example, a playlist matching 450 songs with a 10% limit
returns 45 songs, scaling dynamically as the library grows.

When `limitPercent` is set, refreshSmartPlaylist runs a COUNT query first
to determine the total matching tracks, then resolves the percentage to an
absolute LIMIT before executing the main query. The fixed `limit` field
takes precedence when both are set. Values are clamped to [0, 100] during
JSON unmarshaling.

No database migration is needed since rules are stored as a JSON string.

* fix(criteria): validate percentage limit range in IsPercentageLimit method

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

* fix(criteria): ensure idempotency of ToSql method for expressions

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

---------

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

model/criteria/fields.go

@@ -122,27 +122,24 @@ func mapExpr(expr squirrel.Sqlizer, negate bool, exprFunc func(string, squirrel.
 		log.Fatal(fmt.Sprintf("expr is not a map-based operator: %T", expr))
 	}
 
-	// Extract into a generic map
+	// Extract the field name and value, then build a new map keyed by "value"
+	// for the inner condition. The original map is left untouched so that
+	// ToSql can be called multiple times without corruption.
 	var k string
-	m := make(map[string]any, rv.Len())
+	var v any
 	for _, key := range rv.MapKeys() {
-		// Save the key to build the expression, and use the provided keyName as the key
 		k = key.String()
-		m["value"] = rv.MapIndex(key).Interface()
+		v = rv.MapIndex(key).Interface()
 		break // only one key is expected (and supported)
 	}
 
-	// Clear the original map
-	for _, key := range rv.MapKeys() {
-		rv.SetMapIndex(key, reflect.Value{})
-	}
+	// Create a new map-based expression with "value" as the key, matching the
+	// column name inside json_tree subqueries.
+	newMap := reflect.MakeMap(rv.Type())
+	newMap.SetMapIndex(reflect.ValueOf("value"), reflect.ValueOf(v))
+	newExpr := newMap.Interface().(squirrel.Sqlizer)
 
-	// Write the updated map back into the original variable
-	for key, val := range m {
-		rv.SetMapIndex(reflect.ValueOf(key), reflect.ValueOf(val))
-	}
-
-	return exprFunc(k, expr, negate)
+	return exprFunc(k, newExpr, negate)
 }
 
 // mapTagExpr maps a normal field expression to a tag expression.