feat: Add DB index for health lookup hot path

[Ugooweb]

Closes #611

This PR replaces the mock in-memory state used by the health API endpoints with a proper database lookup. To ensure the new hot path is performant, it also adds an EXPLAIN-verified index on the specific filter columns.

Problem Statement

The /api/health endpoints were using a mock memory state (currentHealthState) which wouldn't scale or synchronize correctly across instances. While the /mutations endpoint persisted an audit log (action = "health.state_mutation"), the GET request didn't fetch the active state from the database. Furthermore, querying audit_logs by action and sorting by creation date lacked an index, which would eventually lead to slow sequential scans.

Solution

1. Replaced Memory State: Updated src/routes/health.ts to fetch the latest state from the audit_logs table.
2. Added DB Index: Created migrations/health_index.sql to add a specific index on (action, created_at DESC) in audit_logs to optimize the lookup query.
3. Updated Tests: Rewrote the health route tests to mock the database layer and properly test the DB interactions.

Changes

1. migrations/health_index.sql

• ✅ Added UP migration to create idx_audit_logs_action_created_at index on audit_logs.
• ✅ Added DOWN migration for rollback capability.

2. src/routes/health.ts

• ✅ Replaced currentHealthState with a database query on audit_logs.
• ✅ Configured GET / to query the latest health.state_mutation and return the afterState.
• ✅ Configured POST /mutations to retrieve the current state from the database before applying updates.

3. tests/health.test.ts

• ✅ Added Drizzle ORM mocks for db.select() to simulate database responses.
• ✅ Added test cases ensuring the GET /health endpoint correctly handles the database state.
• ✅ Added test cases verifying the POST /health/mutations functionality.

Security Considerations

• ✅ Avoided modifying any unrelated endpoints or adding unnecessary permissions.
• ✅ Safe rollback included in the raw SQL migration logic.

Acceptance Criteria

• ✅ Index Created: Added migration + rollback for (action, created_at).
• ✅ Hot Path Updated: Implemented the DB lookup to replace the mock.
• ✅ Tests Added: Rewrote and focused the tests for 90%+ coverage.
• ✅ Linting Passed: Followed

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