Migrate storage from JSON to SQLite with enhanced MVC architecture

- Replace JSON file storage with SQLite database for improved concurrency and data integrity
- Implement repository pattern with dedicated model layer (admin_model, file_model, chunk_model)
- Add database.py with automatic migration from existing JSON files
- Enable WAL mode for thread-safe concurrent access across Gunicorn workers
- Store database files in dedicated db/ folder
- Update controllers to use model layer instead of direct JSON access
- Add admin authentication system with TOTP 2FA support

src/models/__init__.py

@@ -0,0 +1,35 @@
+from .file_metadata import FileMetadata
+
+# Database initialization
+from .database import init_db, get_db_connection, migrate_from_json
+
+# Admin model functions
+from .admin_model import (
+    get_admin,
+    create_admin,
+    admin_exists as model_admin_exists,
+    update_last_login
+)
+
+# File model functions
+from .file_model import (
+    create_file,
+    get_file,
+    increment_download,
+    delete_file as model_delete_file,
+    get_expired_files,
+    cleanup_expired,
+    get_all_files
+)
+
+# Chunk model functions
+from .chunk_model import (
+    create_session,
+    get_session,
+    add_chunk,
+    get_received_chunks,
+    delete_session,
+    get_abandoned_sessions,
+    cleanup_abandoned,
+    update_received_chunks
+)