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
50f33ab900