security: implement CRYPTO-001 and TIMING-001 remediations

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)

PENTEST_PLAN.md

@@ -259,8 +259,8 @@ Testing uses specialized Claude subagents for different security domains, with f
 
 ### Long-term (Low)
 
-- [ ] **CRYPTO-001**: Add certificate serial collision detection
-- [ ] **TIMING-001**: Add constant-time database lookups for sensitive queries
+- [x] **CRYPTO-001**: Add certificate serial collision detection
+- [x] **TIMING-001**: Add constant-time database lookups for sensitive queries
 
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