[anuraggdubey]

Summary

Fixes #93.

The daily submission quota is now enforced atomically per Activity Source:

submit event
  acquire transaction-scoped source lock
  check idempotency key
  count recent submissions
  reject with TOO_MANY_REQUESTS when quota is full
  reserve submission
  release lock

Changes:

• Added a PostgreSQL advisory transaction lock for quota-limited sources.
• Preserved idempotent retries and 409 CONFLICT behavior.
• Added concurrent quota regression coverage.
• Updated the Signing Identity rotation runbook.

Evidence

• Before: Concurrent submissions could pass the quota count before either reservation was inserted.
• After: The quota check and reservation occur under a per-source transaction lock.
• TypeScript check passed.
• Activity ingestion and credential integration tests passed: 21 tests.
• Concurrent submissions correctly allow only the remaining quota and reject excess requests.
• Final diff contains only three intended files.