Auto-detect first certificate issuance and grant admin flag.
Add is_admin column to issued_certificates table.
Add is_admin_certificate() helper function.
Include is_admin in /pki/issue response and X-Is-Admin header in 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
