Add graceful shutdown handler for SIGTERM and SIGINT

[fantaJinMode]

Description

Introduces a registerGracefulShutdown utility that cleanly closes the Fastify server and database pool on process termination signals, preventing in-flight requests and connections from being dropped abruptly.

Benefits

• Clean teardown prevents leaked database connections in containerized environments
• Logs shutdown progress and errors for easier observability
• Simple one-call API makes it easy to adopt in any server setup

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• ♻️ Code refactoring
• ⚡ Performance improvement
• ✅ Test addition or update

Changes Made

• Added registerGracefulShutdown(fastify) function to src/index.ts that handles SIGTERM and SIGINT
• Shutdown sequence closes the Fastify server first, then ends the database pool, then exits with code 0 (or 1 on error)
• Exported db from database.ts to allow pool teardown during shutdown
• Updated examples/basic-server.ts to demonstrate usage of the new function

Testing

• Tests pass locally (npm test)
• New tests added for new functionality
• Existing tests updated (if needed)
• Type checking passes (npm run typecheck)

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have used conventional commit messages (e.g., feat:, fix:)
• I have signed the Contributor License Agreement (the CLA bot will prompt you)

Breaking Changes

None

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/index.ts	0.00%	19 Missing ⚠️

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