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-down fallback.

Also adds docs/dtype-auto-selection.md (planned design: model-native dtype
default read from the checkpoint, precision as explicit override, FA2 fp32
guard) and ignores build artifacts (venv_build.log, .build.pid).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce