feat: add --enable-assets flag, disable assets by default, expose to frontend

Replace --disable-assets-autoscan with --enable-assets so the assets
system (API routes, database sync, background scanning) is off by
default and must be explicitly opted into. Expose the flag as an
"assets" entry in SERVER_FEATURE_FLAGS so the frontend can read it
from GET /features.

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

comfy_api/feature_flags.py

@@ -15,6 +15,7 @@ SERVER_FEATURE_FLAGS: dict[str, Any] = {
     "max_upload_size": args.max_upload_size * 1024 * 1024, # Convert MB to bytes
     "extension": {"manager": {"supports_v4": True}},
     "node_replacements": True,
+    "assets": args.enable_assets,
 }