stage-17: operational hardening — .env keys, model status, backup

Three load-bearing operational pieces before any new features:

* .env.example committed, .env gitignored — per-developer API keys
  (THREATFOX_AUTH_KEY, OTX_API_KEY, NVD_API_KEY) ready for the registrations
  ahead; python-dotenv loads it in the venv CLI; compose picks it up via
  env_file: .env on the cockpit service.

* Cockpit /api/inference-status endpoint + a topbar status chip that polls it
  on page load — "model · live" green when up, "model · offline" amber when
  the inference server is unreachable. No more manual checking. Compose also
  gains a healthcheck on the inference service (applies on next recreate).

* New `psyc backup` command — tars the audit trail (db + sealed packages +
  recipient keys + ledger + datasets) to data/backups/psyc-data-<ts>.tar.gz.
  Excludes the HF model cache, mock-cert receipts, and the re-trainable
  adapters — the goal is the irrecoverable evidence, not bulk artifacts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pyproject.toml

@@ -18,6 +18,7 @@ dependencies = [
     "pynacl>=1.5",
     "structlog>=24.1",
     "sqlalchemy>=2.0",
+    "python-dotenv>=1.0",
 ]
 
 [project.optional-dependencies]