fix(scanner): match library folders only at directory boundaries

A bare string-prefix test (strings.CutPrefix / strings.HasPrefix) let a
registered folder whose path was a string prefix of a sibling claim the
sibling's files: "/app/audiobooks" matched "/app/audiobookssd/Brian
Jacques/...", so getRelativePath sliced the remnant "sd/" into every
stored relative path for that library (prod scan log: "[AUDIO] Created
folder book 'Taggerung' (41 tracks) at sd/Brian Jacques/Redwall/Book 14").
Scanner lookups built their keys with the same function
(getMediaItemByFilePath, scanStateLookup), so writes and lookups were
consistently poisoned and no scan ever corrected them; everything
outside the scanner resolves paths by joining back to the roots
(ResolveMediaPath), so cover/track URLs 404ed for files sitting one
directory away.

The same unchecked HasPrefix pattern also drove five folder-membership
sites, so a sibling-prefix pair misattributed more than paths:
owning-library resolution (processMediaFile, maybeProcessAudioBook),
per-library-type extension allowlisting (isScannableFile), and
folder-structure metadata extraction on both create and update paths —
a file could be scanned under the wrong library's rules entirely.

folderContains(folder, path) is now the single boundary-aware
primitive: path must equal the folder or continue with a
filepath.Separator (after Clean). All seven sites go through it, so no
folder name — however similar to another — can interfere with any
other's files again, in any order of registration; trailing slashes on
registered folders are normalized. Registration order still decides for
genuinely overlapping (nested) registered roots: pre-existing behavior,
deliberately unchanged.

Regression tests (media_scanner_path_test.go, pure unit — no DB) pin the
prod trap (audiobooks/audiobookssd), order-independence, trailing
slashes, mid-word siblings, and the helper itself.

Repair of already-poisoned rows is deliberately separate and safe only
under this fix: a rescan repoints them via the SHA-256 move path
(MoveMediaItemFilePath) once lookups stop producing poisoned keys.

internal/services/media_scanner_audio_multifile.go

@@ -178,7 +178,7 @@ func (s *MediaScanner) maybeProcessAudioBook(ctx context.Context, path string) (
 	// so any scannable audio file belongs to one.
 	rootFolder := ""
 	for _, folder := range s.folders {
-		if strings.HasPrefix(path, folder) {
+		if folderContains(folder, path) {
 			rootFolder = folder
 			break
 		}
@@ -731,7 +731,7 @@ func (s *MediaScanner) rescanMultiFileAudioBook(ctx context.Context, item databa
 
 	rootFolder := ""
 	for _, folder := range s.folders {
-		if strings.HasPrefix(fullPath, folder) {
+		if folderContains(folder, fullPath) {
 			rootFolder = folder
 			break
 		}