Remove legacy key file support after DB migration

- SerieScanner: Remove key file fallback, keep data file fallback
- SystemSettings: Add legacy_key_cleanup_completed flag
- initialization_service: Add cleanup task to remove key files from folders with DB entries
- Tests updated to reflect key file removal from legacy path

Key files caused duplicate key errors on folder rename. DB is now sole source of truth.

src/server/services/initialization_service.py

@@ -1,5 +1,6 @@
 """Centralized initialization service for application startup and setup."""
 import asyncio
+import os
 from pathlib import Path
 from typing import Callable, Optional
 
@@ -122,6 +123,28 @@ async def _mark_legacy_migration_completed() -> None:
     )
 
 
+async def _check_legacy_key_cleanup_status() -> bool:
+    """Check if legacy key file cleanup has been completed.
+
+    Returns:
+        bool: True if cleanup was completed, False otherwise
+    """
+    return await _check_scan_status(
+        check_method=lambda svc, db: svc.is_legacy_key_cleanup_completed(db),
+        scan_type="legacy_key_cleanup",
+        log_completed_msg="Legacy key file cleanup already completed, skipping",
+        log_not_completed_msg="Legacy key file cleanup not yet run, will clean up key files"
+    )
+
+
+async def _mark_legacy_key_cleanup_completed() -> None:
+    """Mark the legacy key file cleanup as completed in system settings."""
+    await _mark_scan_completed(
+        mark_method=lambda svc, db: svc.mark_legacy_key_cleanup_completed(db),
+        scan_type="legacy_key_cleanup"
+    )
+
+
 async def _migrate_legacy_files() -> int:
     """Migrate series from legacy key/data files to database.
 
@@ -151,6 +174,78 @@ async def _migrate_legacy_files() -> int:
         return 0
 
 
+async def _cleanup_legacy_key_files() -> int:
+    """Remove legacy key files from folders that already have DB entries.
+
+    This is a one-time cleanup task that runs at startup after legacy migration.
+    It removes deprecated 'key' files that cause duplicate key errors when
+    folders are renamed, since the DB is now the source of truth.
+
+    Returns:
+        int: Number of key files deleted
+    """
+    from src.server.database.connection import get_db_session
+    from src.server.database.service import AnimeSeriesService
+
+    logger.info("Checking for legacy key files to clean up...")
+
+    if not settings.anime_directory or not os.path.isdir(settings.anime_directory):
+        logger.warning(
+            "Anime directory not configured or does not exist, skipping legacy key cleanup"
+        )
+        return 0
+
+    deleted_count = 0
+    scanned_count = 0
+
+    try:
+        async with get_db_session() as db:
+            # Get all series from DB to know which folders should have key files removed
+            all_series = await AnimeSeriesService.get_all(db)
+
+            # Build a set of known folder names from DB
+            db_folders: set[str] = {series.folder for series in all_series if series.folder}
+
+            for folder_name in db_folders:
+                folder_path = settings.anime_directory / folder_name
+                key_file = folder_path / "key"
+
+                if not key_file.exists():
+                    continue
+
+                scanned_count += 1
+                try:
+                    key_file.unlink()
+                    deleted_count += 1
+                    logger.info(
+                        "Removed legacy key file",
+                        folder=folder_name,
+                        key_file=str(key_file)
+                    )
+                except OSError as exc:
+                    logger.warning(
+                        "Could not remove legacy key file",
+                        folder=folder_name,
+                        key_file=str(key_file),
+                        error=str(exc)
+                    )
+
+    except Exception as e:
+        logger.error(
+            "Legacy key file cleanup failed",
+            error=str(e),
+            exc_info=True
+        )
+        return deleted_count
+
+    logger.info(
+        "Legacy key file cleanup complete",
+        scanned=scanned_count,
+        deleted=deleted_count
+    )
+    return deleted_count
+
+
 async def _sync_anime_folders(progress_service=None) -> int:
     """Scan anime folders and sync series to database.
     
@@ -287,6 +382,13 @@ async def perform_initial_setup(progress_service=None):
         # Sync series from anime folders to database
         await _sync_anime_folders(progress_service)
 
+        # Clean up legacy key files from folders that now have DB entries
+        # This runs after migration/sync to ensure DB entries exist before deletion
+        is_key_cleanup_done = await _check_legacy_key_cleanup_status()
+        if not is_key_cleanup_done:
+            await _cleanup_legacy_key_files()
+            await _mark_legacy_key_cleanup_completed()
+
         # Mark the initial scan as completed
         await _mark_initial_scan_completed()