feat(lib): use new challenge creation flow

Previously Anubis constructed challenge strings from request metadata.
This was a good idea in spirit, but has turned out to be a very bad idea
in practice. This new flow reuses the Store facility to dynamically
create challenge values with completely random data.

This is a fairly big rewrite of how Anubis processes challenges. Right
now it defaults to using the in-memory storage backend, but on-disk
(boltdb) and valkey-based adaptors will come soon.

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/store/decaymap.go

@@ -1,61 +0,0 @@
-package store
-
-import (
-	"context"
-	"fmt"
-	"time"
-
-	"github.com/TecharoHQ/anubis/decaymap"
-)
-
-type decayMapStore struct {
-	store *decaymap.Impl[string, []byte]
-}
-
-func (d *decayMapStore) Delete(_ context.Context, key string) error {
-	if !d.store.Delete(key) {
-		return fmt.Errorf("%w: %q", ErrNotFound, key)
-	}
-
-	return nil
-}
-
-func (d *decayMapStore) Get(_ context.Context, key string) ([]byte, error) {
-	result, ok := d.store.Get(key)
-	if !ok {
-		return nil, fmt.Errorf("%w: %q", ErrNotFound, key)
-	}
-
-	return result, nil
-}
-
-func (d *decayMapStore) Set(_ context.Context, key string, value []byte, expiry time.Duration) error {
-	d.store.Set(key, value, expiry)
-	return nil
-}
-
-func (d *decayMapStore) cleanupThread(ctx context.Context) {
-	t := time.NewTicker(5 * time.Minute)
-	defer t.Stop()
-
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-t.C:
-			d.store.Cleanup()
-		}
-	}
-}
-
-// NewDecayMapStore creates a simple in-memory store. This will not scale
-// to multiple Anubis instances.
-func NewDecayMapStore(ctx context.Context) Interface {
-	result := &decayMapStore{
-		store: decaymap.New[string, []byte](),
-	}
-
-	go result.cleanupThread(ctx)
-
-	return result
-}