Remove migration code and alembic dependency

docs/identifier_standardization_validation.md

@@ -178,10 +178,6 @@ grep -rn "data-key\|data-folder\|data-series" src/server/web/templates/ --includ
     - [ ] All CRUD operations use `key` for identification
     - [ ] Logging uses `key` in messages
 
-3. **`src/server/database/migrations/`**
-    - [ ] Migration files maintain `key` as unique, indexed column
-    - [ ] No migrations that use `folder` as identifier
-
 **Validation Commands:**
 
 ```bash

docs/infrastructure.md

@@ -60,10 +60,9 @@ Throughout the codebase, three identifiers are used for anime series:
 **Valid examples**: `"attack-on-titan"`, `"one-piece"`, `"86-eighty-six"`, `"re-zero"`
 **Invalid examples**: `"Attack On Titan"`, `"attack_on_titan"`, `"attack on titan"`
 
-### Migration Notes
+### Notes
 
 -   **Backward Compatibility**: API endpoints accepting `anime_id` will check `key` first, then fall back to `folder` lookup
--   **Deprecation**: Folder-based lookups are deprecated and will be removed in a future version
 -   **New Code**: Always use `key` for identification; `folder` is metadata only
 
 ## API Endpoints