Add expiration job summary metrics

[awyoo]

Closes #259

Summary

• log a structured expiration-job summary after each run with checked, expired, errors, and durationMs fields
• return the same run counters from expireStaleReservations for callers and tests
• guard individual stale-reservation expiration with warn-level bounty-specific error logging
• add focused Vitest coverage for a run that checks three bounties and expires two

/claim #259

Payout address: 0xe80506A1431B3B4aAca8B260838481deBbdF75Ab

Verification

• git diff --check
• npm --prefix backend test -- reservationExpirationJob.test.ts

Not run: full npm --prefix backend run typecheck is blocked by existing upstream syntax/type errors in src/app.ts, src/index.ts, src/validation/schemas.ts, and metrics exports outside this PR.

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