Feature/process shutdown hygiene

[Mkalbani]

closes #144

Summary

Adds graceful process shutdown hygiene for production while keeping local dev watch behavior intact.

What Changed

• Added SIGTERM and SIGINT shutdown hooks with one-time registration.
• Implemented an idempotent graceful shutdown flow to prevent duplicate teardown.
• Stopped accepting new HTTP requests before shutdown completes.
• Added timeout handling for lingering in-flight connections.
• Closed DB resources during shutdown where applicable (SQLite, PostgreSQL pool, Prisma disconnect).
• Documented production shutdown expectations in README.
• Added minimal tests for graceful shutdown behavior and repeated-signal edge case.

Testing

• npm run lint: pass (warnings only, no new lint errors from this change)
• npm run typecheck: blocked by pre-existing repo issues unrelated to this PR
• npm test: pre-existing failures remain unrelated to this PR
• Focused verification: npx jest index.test.ts --runInBand (passed)

Security / Data Integrity Notes

• Reduces risk of dropped or partial in-flight work during termination.
• Ensures predictable single-run teardown under repeated signals.
• Prevents hung shutdowns by enforcing connection timeout handling.

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