fix: decouple at-rest encryption from JWT_SECRET, add JWT rotation

Introduces a dedicated ENCRYPTION_KEY for encrypting stored secrets
(API keys, MFA TOTP, SMTP password, OIDC client secret) so that
rotating the JWT signing secret no longer invalidates encrypted data,
and a compromised JWT_SECRET no longer exposes stored credentials.

- server/src/config.ts: add ENCRYPTION_KEY (auto-generated to
  data/.encryption_key if not set, same pattern as JWT_SECRET);
  switch JWT_SECRET to `export let` so updateJwtSecret() keeps the
  CJS module binding live for all importers without restart
- apiKeyCrypto.ts, mfaCrypto.ts: derive encryption keys from
  ENCRYPTION_KEY instead of JWT_SECRET
- admin POST /rotate-jwt-secret: generates a new 32-byte hex secret,
  persists it to data/.jwt_secret, updates the live in-process binding
  via updateJwtSecret(), and writes an audit log entry
- Admin panel (Settings → Danger Zone): "Rotate JWT Secret" button
  with a confirmation modal warning that all sessions will be
  invalidated; on success the acting admin is logged out immediately
- docker-compose.yml, .env.example, README, Helm chart (values.yaml,
  secret.yaml, deployment.yaml, NOTES.txt, README): document
  ENCRYPTION_KEY and its upgrade migration path

chart/README.md

@@ -29,5 +29,6 @@ See `values.yaml` for more options.
 ## Notes
 - Ingress is off by default. Enable and configure hosts for your domain.
 - PVCs require a default StorageClass or specify one as needed.
-- JWT_SECRET must be set for production use.
+- `JWT_SECRET` should be set for production use; auto-generated and persisted to the data PVC if not provided.
+- `ENCRYPTION_KEY` encrypts stored secrets (API keys, MFA, SMTP, OIDC) at rest. Auto-generated and persisted to the data PVC if not provided. **Upgrading:** if a previous version used `JWT_SECRET`-derived encryption, set `secretEnv.ENCRYPTION_KEY` to your old `JWT_SECRET` value to keep existing encrypted data readable, then re-save credentials via the admin panel.
 - If using ingress, you must manually keep `env.ALLOWED_ORIGINS` and `ingress.hosts` in sync to ensure CORS works correctly. The chart does not sync these automatically.