feat(auth): replace hand-rolled auth with Ory Kratos — phase 2 backend
Delegates all credential management (login, register, password reset, email verification, social sign-in) to Ory Kratos. The Go API now acts as a resource server: the new KratosAuth middleware validates sessions against the Kratos whoami endpoint, writes the local User mirror into Echo context, and all existing domain handlers continue working unchanged. Hand-rolled token auth, AuthToken model, apple_auth/ google_auth services, and the auth refresh flow are removed. Tests are updated to use the fake-token middleware pattern so existing integration assertions require no rewrite. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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