Add GET /oauth/providers endpoint for OAuth provider availability

[Copilot]

BFF/frontend has no way to know whether OAuth providers are configured before rendering login options, leading to broken "Login with GitHub" buttons when OAuth is misconfigured.

Changes

• Proto: Added ListOAuthProviders RPC mapped to GET /oauth/providers with OAuthProviderStatus message type (name, enabled, optional reason)
• Service: Implemented config validation — checks client_id and client_secret presence, returns machine-readable disable reasons
• Generated code: Regenerated gRPC server/client, gateway handlers, and OpenAPI spec via protoc
• Tests: Four unit tests covering enabled, missing_client_id, missing_client_secret, and completeness of provider list

Response shape

{"providers": [{"name": "github", "enabled": true}]}

{"providers": [{"name": "github", "enabled": false, "reason": "missing_client_id"}]}

Providers are sorted alphabetically for deterministic ordering.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• buf.build
  • Triggering command: /home/REDACTED/go/bin/buf buf generate 4Kwm452TDXZMOUiLiUG4/4Kwm452TDXZMOUiLiUG4 l/linux_amd64/compile go1.25.9 -c=4 -nolocalimports -importcfg /tmp/go-build1844953333/b748/importcfg (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)