feat: add SIGINT graceful shutdown

[Majormaxx]

Summary

Handles SIGINT (Ctrl+C) the same way as the existing SIGTERM handler: stops the indexer, closes the HTTP server, and logs before exiting. A shared shutdown() function eliminates duplication between the two signal handlers.

Changes — backend/src/index.ts

• Extracted a shutdown() function that logs "Shutting down", calls indexer.stop(), and closes the server
• Registered shutdown for both SIGTERM and SIGINT

Changes — backend/src/api/controllers/vaults.ts

• Fixed pre-existing lint error: renamed err → _err in two unused catch variables (required for CI lint to pass)

Acceptance criteria

• Pressing Ctrl+C logs "Shutting down" and exits cleanly
• SIGINT handled identically to SIGTERM

Test plan

cd backend
npm run lint   # ✓ 0 errors
npm run build  # ✓ no TS errors
npm run test   # ✓ 55/55 tests pass

Closes #511

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