CRYPTO-001: Certificate serial collision detection
- Add _generate_unique_serial() helper for database-backed PKI
- Add _generate_unique_serial() method for in-memory PKI class
- Check database for existing serial before certificate issuance
- Retry with new random serial if collision detected (max 5 attempts)
TIMING-001: Constant-time database lookups for sensitive queries
- Add dummy PBKDF2 verification when paste not found
- Prevents timing-based enumeration (attackers can't distinguish
'not found' from 'wrong password' by measuring response time)