feat(cli): add pls export/import subcommands for bulk playlist management (#5412)

* refactor: rename ImportFile to ImportFromFolder in playlists service

* feat: add ImportFile method with library/folder resolution

* feat: allow sync flag upgrade on re-import of non-synced playlists

* feat: add pls export subcommand with bulk and single export

Add `navidrome pls export` command that supports:
- Single playlist export to stdout (-p flag only)
- Single playlist export to directory (-p and -o flags)
- Bulk export of all playlists to a directory (-o flag only)
- Filtering by user (-u flag)
- Automatic filename sanitization and collision detection

Also extracts findPlaylist helper from runExporter for reuse.

* feat: add pls import subcommand with sync flag support

* fix: improve error message for export without output directory

* test: add tests for ImportFile sync flag and sync upgrade behavior

* refactor: streamline export and import logic by removing redundant comments and improving library path matching

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

* feat: update ImportFile method to include sync flag for playlist imports

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

* feat: implement fetchPlaylists function to streamline playlist retrieval

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

* feat: replace inline filename sanitization with centralized utility function

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

* feat: refactor playlist import logic to consolidate sync handling and improve method signatures

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

* fix: address code review feedback on playlist import/export

- Fix duplicate playlist creation on non-sync re-import: only reconcile
  sync flag when the playlist was actually persisted (has an ID)
- Distinguish "not in any library" from real errors in resolveFolder
  using a sentinel error, so DB/folder errors propagate instead of
  falling back to ImportM3U
- Use bufio.Scanner in countM3UTrackLines instead of reading entire file

* feat: replace bufio.Scanner with UTF8Reader and LinesFrom utility for improved file reading

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

* fix: record path for outside-library imports to prevent duplicates

Files outside all libraries now go through updatePlaylist with the
absolute path recorded, so re-importing the same file updates the
existing playlist instead of creating a duplicate.

* refactor: name guard condition in updatePlaylist for readability

Extracted the compound boolean expression into a named local variable
`alreadyImportedAndNotSynced` to make the intent of the early-return
guard clearer at a glance.

* add godocs

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

---------

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

scanner/phase_4_playlists.go

@@ -100,7 +100,7 @@ func (p *phasePlaylists) processPlaylistsInFolder(folder *model.Folder) (*model.
 			continue
 		}
 		// BFR: Check if playlist needs to be refreshed (timestamp, sync flag, etc)
-		pls, err := p.pls.ImportFile(p.ctx, folder, f.Name())
+		pls, err := p.pls.ImportFromFolder(p.ctx, folder, f.Name())
 		if err != nil {
 			continue
 		}

scanner/phase_4_playlists_test.go

@@ -97,9 +97,9 @@ var _ = Describe("phasePlaylists", func() {
 			_ = os.WriteFile(file1, []byte{}, 0600)
 			_ = os.WriteFile(file2, []byte{}, 0600)
 
-			pls.On("ImportFile", mock.Anything, folder, "playlist1.m3u").
+			pls.On("ImportFromFolder", mock.Anything, folder, "playlist1.m3u").
 				Return(&model.Playlist{}, nil)
-			pls.On("ImportFile", mock.Anything, folder, "playlist2.m3u").
+			pls.On("ImportFromFolder", mock.Anything, folder, "playlist2.m3u").
 				Return(&model.Playlist{}, nil)
 
 			_, err := phase.processPlaylistsInFolder(folder)
@@ -134,7 +134,7 @@ type mockPlaylists struct {
 	playlists.Playlists
 }
 
-func (p *mockPlaylists) ImportFile(ctx context.Context, folder *model.Folder, filename string) (*model.Playlist, error) {
+func (p *mockPlaylists) ImportFromFolder(ctx context.Context, folder *model.Folder, filename string) (*model.Playlist, error) {
 	args := p.Called(ctx, folder, filename)
 	return args.Get(0).(*model.Playlist), args.Error(1)
 }