feat(api): add anchor flows, admin ops routes, and graceful shutdown

[ikemHood]

Summary

This PR implements three backend tasks for anchor operations, admin/internal support routes, and graceful shutdown behavior.

What’s included

• Added protected anchor flow endpoints:
  • POST /api/anchor/deposit
  • POST /api/anchor/withdraw
  • Returns 501 when anchor integration is not configured.
• Added protected admin routes under /api/admin/* with X-Admin-Key / admin cookie auth:
  • POST /api/admin/cache/clear
  • GET /api/admin/users
  • GET /api/admin/audit
• Added graceful shutdown handling for in-process background jobs:
  • Handles SIGTERM / SIGINT
  • Stops new background work
  • Waits for in-flight jobs with timeout before exit
• Updated docs/env/openapi for new routes and configuration.

Validation

• npm run lint
• npm run test
• npm run test:integration

Issues to close

Closes #170
Closes #199
Closes #201

[Baskarayelu]

please resolve the conflicts

[Baskarayelu]

@ikemHood Can you please resolve the conflicts?

[ikemHood]

@Baskarayelu