Add request ID middleware

[Martins-O]

Title: feat(backend): add request ID middleware
Description:
Adds a lightweight Express middleware that generates a UUID per request, creates a pino child logger scoped to that request, and sets the X-Request-ID response header.
Changes:

• backend/src/api/middleware/requestId.ts — new middleware: generates crypto.randomUUID(), creates logger.child({ requestId }), sets X-Request-ID header, augments Express Request type with requestId and log properties
• backend/src/app.ts — registers middleware after CORS, before all route handlers
• backend/src/api/middleware/errors.ts — uses req.log (with fallback) so error logs include the request ID
  Branch name note: The checklist specifies issue--. This branch is add-request-id-middleware. If you want to rename it, I can help with git branch -m.
  Testing: Typecheck passes (tsc --noEmit clean). Pre-existing lint warnings in vaults.ts (2x unused err in catch blocks) are unrelated.

Closes #513

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