Add data file to database sync functionality

- Add get_all_series_from_data_files() to SeriesApp
- Sync series from data files to DB on startup
- Add unit tests for new SeriesApp method
- Add integration tests for sync functionality
- Update documentation

src/core/SeriesApp.py

@@ -599,3 +599,56 @@ class SeriesApp:
             looks up series by their unique key, not by folder name.
         """
         return self.list.get_by_key(key)
+
+    def get_all_series_from_data_files(self) -> List[Serie]:
+        """
+        Get all series from data files in the anime directory.
+
+        Scans the directory_to_search for all 'data' files and loads
+        the Serie metadata from each file. This method is synchronous
+        and can be wrapped with asyncio.to_thread if needed for async
+        contexts.
+
+        Returns:
+            List of Serie objects found in data files. Returns an empty
+            list if no data files are found or if the directory doesn't
+            exist.
+
+        Example:
+            series_app = SeriesApp("/path/to/anime")
+            all_series = series_app.get_all_series_from_data_files()
+            for serie in all_series:
+                print(f"Found: {serie.name} (key={serie.key})")
+        """
+        logger.info(
+            "Scanning for data files in directory: %s",
+            self.directory_to_search
+        )
+
+        # Create a fresh SerieList instance for file-based loading
+        # This ensures we get all series from data files without
+        # interfering with the main instance's state
+        try:
+            temp_list = SerieList(
+                self.directory_to_search,
+                db_session=None,  # Force file-based loading
+                skip_load=False  # Allow automatic loading
+            )
+        except Exception as e:
+            logger.error(
+                "Failed to scan directory for data files: %s",
+                str(e),
+                exc_info=True
+            )
+            return []
+
+        # Get all series from the temporary list
+        all_series = temp_list.get_all()
+
+        logger.info(
+            "Found %d series from data files in %s",
+            len(all_series),
+            self.directory_to_search
+        )
+
+        return all_series