Bundle the whisper base model + one-shot standalone provisioning

- models/hf-cache/ vendors Systran/faster-whisper-base (142MB, gitignored);
  engine spawn pins HF_HUB_CACHE there, so transcription works with a cold
  ~/.cache and fully offline
- verified: HOME=/tmp/fakehome HF_HUB_OFFLINE=1 model loads and transcribes
  via the bundled cache alone
- setup-standalone.sh: idempotent fresh-clone provisioning (local venv,
  model copy-or-download, engine boot smoke on :8123) — runs green
- backend/README rewritten for the desktop bundled-lite reality (was
  pointing at a nonexistent root README/docs and a Postgres quickstart)

Kotlin suite still green (gradlew test, 0 failures)

.gitignore

@@ -7,3 +7,4 @@ backend/__pycache__/
 **/__pycache__/
 *.pyc
 .dev-restart-flag
+models/hf-cache/