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>

utils/str/sanitize_strings.go

@@ -52,6 +52,22 @@ func SanitizeHTML(text string) string {
 	return policy.Sanitize(html.UnescapeString(text))
 }
 
+var filenameReplacer = strings.NewReplacer(
+	"/", "_",
+	"\\", "_",
+	":", "_",
+	"*", "_",
+	"?", "_",
+	"\"", "_",
+	"<", "_",
+	">", "_",
+	"|", "_",
+)
+
+func SanitizeFilename(name string) string {
+	return filenameReplacer.Replace(name)
+}
+
 func SanitizeFieldForSorting(originalValue string) string {
 	v := strings.TrimSpace(sanitize.Accents(originalValue))
 	return Clear(strings.ToLower(v))