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

app/assets/api/routes.py

@@ -54,7 +54,7 @@ def _require_assets_feature_enabled(handler):
             return _build_error_response(
                 503,
                 "SERVICE_DISABLED",
-                "Assets system is disabled. Start the server with --enable-assets to use this feature.",
+                "Assets system is unavailable.",
             )
         return await handler(request)