Development-signed iOS apps cannot enroll with the current gateway
because their Apple App Attest AAGUID is rejected.
Add an explicit personal development build feature and environment
setting so a development gateway can use a developer's own application
identity and sandbox APNs credentials.
Ordinary gateway builds remain production-only. Special builds also
default to production, enforce exactly the selected signed AAGUID, and
retain certificate-chain, pinned-root, nonce, app-ID, key,
credential-ID, and counter verification.
Document the matching mobile signing configuration and expose the
feature through an optional Docker build argument.
### Validation
The development image deployed successfully to a personal Railway stack;
private readiness, enrollment challenges, authenticated relay access,
and APNs sandbox TLS/HTTP2 connectivity passed.
Signed-off-by: Tom Brow <tomb@block.xyz>
