feat(scanner): add DB fallback for series key resolution

When SerieScanner encounters a folder without a local key or data file,
it now optionally falls back to a database lookup by folder name. This
prevents newly-added series from being silently skipped on rescan when
their metadata only lives in the DB.

Changes:
- SerieScanner accepts an optional db_lookup callable
- SeriesApp forwards db_lookup to SerieScanner
- AnimeSeriesService adds get_by_folder_sync() helper
- dependencies.py wires a sync DB lookup into get_series_app()
- Unit tests cover fallback hit, miss, and exception paths

src/core/SerieScanner.py

@@ -15,7 +15,7 @@ import os
 import re
 import traceback
 import uuid
-from typing import Iterable, Iterator, Optional
+from typing import Callable, Iterable, Iterator, Optional
 
 from events import Events
 
@@ -43,12 +43,17 @@ class SerieScanner:
         scanner = SerieScanner("/path/to/anime", loader)
         scanner.scan()
         # Results are in scanner.keyDict
+
+        # With DB lookup fallback:
+        scanner = SerieScanner("/path/to/anime", loader,
+                               db_lookup=lambda folder: my_db.get_by_folder(folder))
     """
 
     def __init__(
         self,
         basePath: str,
         loader: Loader,
+        db_lookup: Optional[Callable[[str], Optional["Serie"]]] = None,
     ) -> None:
         """
         Initialize the SerieScanner.
@@ -56,8 +61,12 @@ class SerieScanner:
         Args:
             basePath: Base directory containing anime series
             loader: Loader instance for fetching series information
-            callback_manager: Optional callback manager for progress updates
-            
+            db_lookup: Optional callable ``(folder_name) -> Serie | None``.
+                When provided, it is called as a fallback when neither a
+                ``key`` file nor a ``data`` file is found in the folder.
+                This allows the database to supply the series key for
+                folders that have never had a local key file.
+
         Raises:
             ValueError: If basePath is invalid or doesn't exist
         """
@@ -75,6 +84,7 @@ class SerieScanner:
         self.directory: str = abs_path
         self.keyDict: dict[str, Serie] = {}
         self.loader: Loader = loader
+        self._db_lookup: Optional[Callable[[str], Optional[Serie]]] = db_lookup
         self._current_operation_id: Optional[str] = None
         self.events = Events()
         
@@ -268,6 +278,30 @@ class SerieScanner:
                     )
 
                     serie = self.__read_data_from_file(folder)
+                    if serie is None or not serie.key or not serie.key.strip():
+                        # Fallback: ask the database for a matching series
+                        if self._db_lookup is not None:
+                            try:
+                                serie = self._db_lookup(folder)
+                                if serie:
+                                    logger.info(
+                                        "DB lookup resolved folder '%s' -> key='%s'",
+                                        folder,
+                                        serie.key,
+                                    )
+                            except Exception as exc:
+                                logger.warning(
+                                    "DB lookup failed for folder '%s': %s",
+                                    folder,
+                                    exc,
+                                )
+                                serie = None
+
+                    if serie is None or not serie.key or not serie.key.strip():
+                        logger.warning(
+                            "No key or data file found for folder '%s', skipping",
+                            folder,
+                        )
                     if (
                         serie is not None
                         and serie.key