The models page reads /admin/api/cached-models (served by the system worker),
whose _scan_caches() merges each model's saved config (precision/quant/ctx) into
the cards. The scan freshness check _scan_signature() only fingerprinted the
HF/GGUF cache directories, never models.json, and model-configure never
invalidated the scan. A config save rewrites models.json but touches no cache
file, so the signature stayed unchanged and the scan was served stale for the
full TTL (600s) -- the engine reloaded but the front showed old config until a
full restart.

- _scan_signature(): fold models.json mtime into the signature so any process
  serving cached-models re-scans on the next read after a save.
- system worker /internal/reload-config: call _invalidate_cache_scan() after
  rebinding config, so the front's reload-push refreshes the scan immediately.
- engine /internal/reload-config: same invalidation for single-process /
  system-worker-do...