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

server/src/services/mfaCrypto.ts

@@ -1,8 +1,8 @@
 import crypto from 'crypto';
-import { JWT_SECRET } from '../config';
+import { ENCRYPTION_KEY } from '../config';
 
 function getKey(): Buffer {
-  return crypto.createHash('sha256').update(`${JWT_SECRET}:mfa:v1`).digest();
+  return crypto.createHash('sha256').update(`${ENCRYPTION_KEY}:mfa:v1`).digest();
 }
 
 /** Encrypt TOTP secret for storage in SQLite. */