feat(api): Add ETag / 304 caching on /api/maintenance

[kosisochukwu1234]

Closes #886

This PR addresses the backend issue for the GrantFox FWC26 campaign. It introduces strong ETag generation and HTTP 304 Not Modified support for the public-facing /api/maintenance endpoint.

Previously, the endpoint returned a full 200 OK response along with the payload every time it was polled. With this update, we emit a strong SHA-256 ETag corresponding to the response payload. By honoring the If-None-Match header in subsequent client requests, we cut down bandwidth significantly on repeat reads, meeting the campaign's high-traffic performance needs.

Changes Included

• Middleware Integration: Added etagMiddleware to the publicMaintenanceRouter in src/routes/maintenance.ts.
• Documentation: Updated the JSDoc comments in src/routes/maintenance.ts to document the new ETag generation and caching behavior, making the API/visible changes clear to other contributors.
• Testing:
  • Added targeted integration test suites in src/routes/maintenance.test.ts under describe('ETag / 304 caching (issue #021)').
  • Verified that a strong ETag is present on first requests.
  • Verified that requests carrying a matching If-None-Match correctly short-circuit and return 304 Not Modified with an empty body and stripped content headers.
  • Verified that mismatched If-None-Match strings correctly fall back to a standard 200 OK response.

Security & Reliability

• Validated that the CORS allowance is strictly honored alongside the caching mechanism.
• Kept dependencies unchanged.
• Adhered to the existing repository code style and lint rules.

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[greatest0fallt1me]

All checks green — merging. Nice job! 🚀