feat: implement NIP-09 with hard delete

Implement event deletion (NIP-09) using hard delete approach:
- Kind 5 events trigger deletion but are not stored themselves
- ProcessDeletion hard deletes referenced events (DELETE FROM events)
- Only authors can delete their own events (pubkey verification)
- Support multiple event IDs in single deletion request
- No deletions table needed (simpler schema)
- Added 4 deletion tests covering various scenarios
- All 45 tests passing

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