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

.gitignore

@@ -31,6 +31,7 @@ data/*.db-shm
 .credentials.json
 *.pem
 *.key
+keys/
 
 # Build
 dist/