refactor: add folder rename configuration and service

Add configurable folder rename patterns via settings with anime_folder_rename_regex and custom_pattern options. Integrate into SerieScanner and SeriesApp for consistent episode organization.

src/server/services/folder_rename_service.py

@@ -596,7 +596,50 @@ async def validate_and_rename_series_folders(dry_run: bool = False) -> Dict[str,
                 current_name,
                 expected_name,
             )
-            stats["errors"] += 1
+            # Target folder exists — remove source folder and delete its DB record
+            # (target folder's DB record survives, source folder's record must be removed
+            #  to avoid orphaning episodes/downloads)
+            try:
+                import shutil
+
+                shutil.rmtree(series_dir)
+                logger.info(
+                    "Removed source folder '%s' — series already exists at target",
+                    current_name,
+                )
+
+                # Delete source DB record (cascades to episodes and download items)
+                async with get_db_session() as db:
+                    source_series = await AnimeSeriesService.get_by_key(db, current_name)
+                    if source_series is None:
+                        # Fallback: find by folder name
+                        all_series = await AnimeSeriesService.get_all(db)
+                        for s in all_series:
+                            if s.folder == current_name:
+                                source_series = s
+                                break
+                    if source_series is not None:
+                        await AnimeSeriesService.delete(db, source_series.id)
+                        logger.info(
+                            "Deleted source DB record for '%s' (id=%s) — target folder '%s' retains DB record",
+                            current_name,
+                            source_series.id,
+                            expected_name,
+                        )
+                    else:
+                        logger.info(
+                            "No DB record found for source folder '%s' — folder removed only",
+                            current_name,
+                        )
+
+                stats["renamed"] += 1
+            except OSError as exc:
+                logger.error(
+                    "Failed to remove source folder '%s': %s",
+                    current_name,
+                    exc,
+                )
+                stats["errors"] += 1
             continue
 
         # Check path length limits