fix(health): add pagination metadata envelope on /api/health

[ugoocreates-pixel]

Closes #901

This PR addresses the backend issue for the GrantFox FWC26 campaign. It updates the /api/health endpoint to return responses wrapped in a pagination metadata envelope ({items, next_cursor, total?}). This provides a standardized response structure, removing any ambiguity for clients consuming the API.

Changes Made

• Endpoint Update (src/routes/health.ts):
  • Refactored the standard health check response to return the system health payload inside the items array of the envelope.
  • Set next_cursor to null and total to 1 by default.
  • Refactored the maintenance mode response to be wrapped in the same pagination envelope format.
• Testing Updates (tests/integration/health.test.ts):
  • Updated 50+ existing assertions to validate against the new response.body.items[0] structure instead of the root response.body.
  • Added a dedicated test to verify that the envelope structure itself (items array, next_cursor, total) is correctly formed.

Requirements Addressed

• Return {items, next_cursor, total?} envelope on /api/health.
• Add focused tests for the envelope format.
• Ensure minimum 90% test coverage on changed lines (tested via unit/integration coverage).
• Maintain existing input validation and standardized error patterns.
• Adhere to the repo's linting and formatting rules.

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[greatest0fallt1me]

LGTM — clean, tested. Merged 🙌