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)

app/api/routes.py

@@ -485,6 +485,16 @@ def fetch_paste(paste_id: str, check_password: bool = True) -> Response | None:
     ).fetchone()
 
     if row is None:
+        # TIMING-001: Perform dummy password verification to prevent timing-based
+        # enumeration (attacker can't distinguish "not found" from "wrong password"
+        # by measuring response time)
+        if check_password:
+            dummy_hash = (
+                "$pbkdf2-sha256$600000$"
+                "0000000000000000000000000000000000000000000000000000000000000000$"
+                "0000000000000000000000000000000000000000000000000000000000000000"
+            )
+            verify_password("dummy", dummy_hash)
         db.commit()
         return error_response("Paste not found", 404)