Fix MTGJSON download functionality

- Add refresh router with admin-only endpoints
- Fix _decompress_file to handle .psql files correctly
- Fix duplicate imports in mtgjson_manager.py
- Mount refresh router in main.py
- Add download_mtgjson.py standalone script
- Add SUPPORTED_FILE_TYPES.md documentation

backend/app/routers/__init__.py

@@ -12,6 +12,7 @@ from app.routers import games
 from app.routers import admin
 from app.routers import card_router
 from app.routers import interactions
+from app.routers import refresh
 
 __all__ = [
     "auth",
@@ -22,4 +23,5 @@ __all__ = [
     "admin",
     "card_router",
     "interactions",
+    "refresh",
 ]