fix(security): resolve audit findings — secrets, env contract, migrations

Removes hardcoded fallback secrets and makes a misconfigured deploy fail
loudly instead of silently falling back to development defaults.

- Remove insecure JWT fallback secrets (messages.module, configuration)
- Remove the 'default-secret' fallback for the 2FA TOTP encryption key and
  allow a dedicated TWO_FACTOR_ENCRYPTION_KEY so rotating JWT_SECRET no
  longer locks out every 2FA user (see docs/2fa-key-rotation.md)
- Require EMAIL_API_URL; drop the hardcoded vendor email endpoint
- Drive WebSocket CORS from CORS_ORIGINS instead of origin:'*'
- Load .env before any Nest module is imported (src/load-env.ts). Decorator
  arguments evaluate at import time, so the gateway previously froze its CORS
  config to the localhost fallback even when CORS_ORIGINS was set
- Add boot-time env validation: missing required vars, weak JWT_SECRET, and
  inverted access/refresh token lifetimes now abort startup
- Enable Redis TLS certificate verification
- Require ADMIN_EMAIL/ADMIN_PASSWORD for the seed; remove the published
  default super-admin credentials and stop printing them
- Add the initial Prisma migration and stop gitignoring prisma/migrations
- Make .env.example an accurate configuration contract (admin bootstrap,
  REDIS_TLS, S3_ENDPOINT, 2FA key, Firebase path; drop the dead SMTP block)
- Add handover documentation: architecture, ER model, sequence and data-flow
  diagrams, 2FA key rotation runbook

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

prisma/migrations/migration_lock.toml

@@ -0,0 +1,3 @@
+# Please do not edit this file manually
+# It should be added in your version-control system (e.g., Git)
+provider = "postgresql"