Add GitHub OAuth login with provider registry

Implement OAuth 2.0 login via GitHub as an alternative to email/password.
Uses a provider registry pattern (internal/auth/oauth/) so adding Google
or other providers later requires only a new Provider implementation.

Flow: GET /v1/auth/oauth/github redirects to GitHub, callback exchanges
the code for a user profile, upserts the user + team atomically, and
redirects to the frontend with a JWT token.

Key changes:
- Migration: make password_hash nullable, add oauth_providers table
- Provider registry with GitHubProvider (profile + email fallback)
- CSRF state cookie with HMAC-SHA256 validation
- Race-safe registration (23505 collision retries as login)
- Startup validation: CP_PUBLIC_URL required when OAuth is configured

Not fully tested — needs integration tests with a real GitHub OAuth app
and end-to-end testing with the frontend callback page.

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/vishvananda/netlink v1.1.1-0.20210330154013-f5de75959ad5
 	github.com/vishvananda/netns v0.0.0-20210104183010-2eb08e3e575f
 	golang.org/x/crypto v0.49.0
+	golang.org/x/oauth2 v0.36.0
 	golang.org/x/sys v0.42.0
 	google.golang.org/protobuf v1.36.11
 )