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/core/SerieScanner.py

@@ -618,7 +618,8 @@ class SerieScanner:
         1. Query DB by folder name
         2. If found, return cached Serie object
         3. If not in DB, fall back to provider search via _db_lookup callback
-        4. (Legacy) If still not found, try reading 'key' file as last resort
+        4. If still not found, try reading 'data' file for legacy deployments
+        5. Generate key from folder name as last resort
 
         Args:
             folder_name: Filesystem folder name
@@ -627,9 +628,8 @@ class SerieScanner:
             Serie object with valid key if found, None otherwise
 
         Note:
-            DB is the source of truth. File-based lookups (key/data files)
-            are temporary backward compatibility for deployments with old data.
-            Will be removed in v3.0.0.
+            DB is the source of truth. File-based lookups (data files)
+            are temporary backward compatibility for CLI-only deployments.
         """
         # Step 1: Try DB lookup by folder name
         try:
@@ -680,25 +680,8 @@ class SerieScanner:
                     exc
                 )
 
-        # Step 3: Legacy fallback - TEMPORARY (remove in v3.0.0)
+        # Step 3: Legacy data file fallback (CLI-only deployments)
         folder_path = os.path.join(self.directory, folder_name)
-        key_file = os.path.join(folder_path, 'key')
-        if os.path.exists(key_file):
-            logger.warning(
-                "Using legacy 'key' file for '%s' - this fallback is deprecated "
-                "and will be removed in v3.0.0",
-                folder_name
-            )
-            with open(key_file, 'r', encoding='utf-8') as file:
-                key = file.read().strip()
-                logger.info(
-                    "Key found for folder '%s': %s",
-                    folder_name,
-                    key
-                )
-                year_from_folder = self._extract_year_from_folder_name(folder_name)
-                return Serie(key, "", "aniworld.to", folder_name, dict(), year=year_from_folder)
-
         serie_file = os.path.join(folder_path, 'data')
         if os.path.exists(serie_file):
             with open(serie_file, "rb") as file: