Remove ALLOW_REGISTRATION env var — managed via admin UI instead

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

backend/main.py

@@ -133,10 +133,7 @@ def on_startup():
                 db.commit()
         # Seed system_config from env if not already set
         if not db.query(SystemConfig).filter_by(key="allow_registration").first():
-            db.add(SystemConfig(
-                key="allow_registration",
-                value="true" if settings.allow_registration else "false",
-            ))
+            db.add(SystemConfig(key="allow_registration", value="true"))
             db.commit()
 
         # Apply user's saved concurrent download limit on startup