add /register endpoint for public certificate registration

Public endpoint allows anyone to obtain a client certificate for
authentication. Features:

- Higher PoW difficulty than paste creation (24 vs 20 bits)
- Auto-generates CA on first registration if not present
- Returns PKCS#12 bundle with cert, key, and CA
- Configurable via FLASKPASTE_REGISTER_POW

Endpoints:
- GET /register/challenge - Get registration PoW challenge
- POST /register - Register and receive PKCS#12 bundle

app/config.py

@@ -63,6 +63,8 @@ class Config:
     POW_CHALLENGE_TTL = int(os.environ.get("FLASKPASTE_POW_TTL", "300"))  # 5 minutes
     # Secret key for signing challenges (auto-generated if not set)
     POW_SECRET = os.environ.get("FLASKPASTE_POW_SECRET", "")
+    # Registration PoW difficulty (higher than paste creation for security)
+    REGISTER_POW_DIFFICULTY = int(os.environ.get("FLASKPASTE_REGISTER_POW", "24"))
 
     # Anti-flood: dynamically increase PoW difficulty under load
     ANTIFLOOD_ENABLED = os.environ.get("FLASKPASTE_ANTIFLOOD", "1").lower() in (