feat(assets): enable asset system by default; remove --enable-assets

The asset system is now always on. Removes the --enable-assets opt-in
flag and the conditional registration it gated:

- Asset API routes are always registered (with the user manager)
- The background asset scanner always runs (hashing stays opt-in via
  --enable-asset-hashing, default off — no new blake3 cost on a default launch)
- /upload/image asset registration and output enrichment always active
- The "assets" server feature flag is now unconditionally true
- A database-init failure degrades gracefully instead of hard-exiting,
  matching the prior default-path behavior

comfy_api/feature_flags.py

@@ -103,7 +103,7 @@ _CORE_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,
+    "assets": True,
 }
 
 # CLI-provided flags cannot overwrite core flags