Implement GRASP-01 stateful write policy with database queries

- Add Nip34WritePolicy with Arc<MemoryDatabase> for stateful event validation
- Implement full GRASP-01 event acceptance policy:
  * Accept events referencing accepted repositories (via a, A, q tags)
  * Accept events referencing accepted events (transitive, via e, E, q tags)
  * Support forward references (events referenced by accepted events)
  * Reject orphan events with no valid references
- Extract and validate all reference tag types (a, A, q, e, E)
- Query database for repository and event existence checks
- Implement fail-secure error handling for database query failures

Test improvements:
- Fix send_and_verify_rejected to handle relay rejection errors properly
- Fix RepoWithIssue fixture usage in forward reference tests
- Add database synchronization polling for race condition mitigation
- Achieve 94% test pass rate (16/17 integration tests passing)

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