Move client key to database with admin UI for key management and QR code generation

- Store client key in SQLite database instead of environment variable
- Add database migration from CLIENT_KEY environment variable to preserve existing keys
- Add admin UI with tabbed interface (Configuration and QR Code tabs)
- Implement QR code generation containing server config (address, port, key) for Android app
- Add functionality to regenerate client key with warning dialog
- Add buttons to download QR code as PNG and copy QR image to clipboard
- Add manual input fields for server address and port configuration
- Update requirements.txt with pyotp and qrcode dependencies

src/models/__init__.py

@@ -1,7 +1,7 @@
 from .file_metadata import FileMetadata
 
 # Database initialization
-from .database import init_db, get_db_connection, migrate_from_json
+from .database import init_db, get_db_connection, migrate_from_json, migrate_client_key_to_db
 
 # Admin model functions
 from .admin_model import (
@@ -33,3 +33,13 @@ from .chunk_model import (
     cleanup_abandoned,
     update_received_chunks
 )
+
+# Config model functions
+from .config_model import (
+    get_config,
+    get_client_key,
+    init_config,
+    update_server_address,
+    update_server_port,
+    regenerate_client_key
+)