feat: cache warm audit

[Derekwalter999]

Description

This PR addresses backend issue #385 for the GrantFox FWC26 campaign (buffer2 #2). It implements an automated cache warm-up sequence for the /api/audit endpoint during application startup, which effectively mitigates cold-cache latency spikes for initial requests.

Changes Made

• src/services/auditCacheWarm.ts: Added the warmAuditCache() service function. It utilizes the native Node fetch API to make an internal background request to /api/audit?limit=10 with a timeout mechanism.
• src/index.ts: Wired the cache warm-up logic into the server lifecycle, executing it immediately after the Express app binds and begins listening.
• src/__tests__/services/auditCacheWarm.test.ts: Added a comprehensive test suite to validate success paths, non-200 responses, and network-level fetch errors.

Compliance & Acceptance Criteria

• Implementation matches description: The /api/audit endpoint is now actively warmed up on startup.
• Test Coverage: Exceeds the 90% minimum requirement for changed lines, with tests fully covering the new cache warm service.
• Structured Logging: Includes correlation IDs (uuidv4()) and adheres to the standardized Pino logging structure for tracking warm-up lifecycles.
• Documentation: Added clear inline TSDoc comments for the new service to explain its purpose and behavior.
• Error Boundaries: Request failures are gracefully caught, logged, and isolated so they do not crash the server startup process.

Closes #385

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