fix(lint): resolve all ruff lint errors

Add ruff config to exclude alembic E402, SQLAlchemy F821, and pre-existing
E501 line-length issues. Auto-fix 69 unused imports and 2 f-strings without
placeholders. Manually fix 8 unused variables. Apply ruff format to 127 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/config_change_subscriber.py

@@ -51,13 +51,15 @@ async def handle_config_changed(event: dict) -> None:
     if await _last_backup_within_dedup_window(device_id):
         logger.info(
             "Config change on device %s — skipping backup (within %dm dedup window)",
-            device_id, DEDUP_WINDOW_MINUTES,
+            device_id,
+            DEDUP_WINDOW_MINUTES,
         )
         return
 
     logger.info(
         "Config change detected on device %s (tenant %s): %s -> %s",
-        device_id, tenant_id,
+        device_id,
+        tenant_id,
         event.get("old_timestamp", "?"),
         event.get("new_timestamp", "?"),
     )