feat(auth): add Google OAuth callback with oauth_identities linking

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/oauth.py

@@ -0,0 +1,13 @@
+from pydantic import BaseModel
+
+
+class OAuthCallbackPayload(BaseModel):
+    code: str
+    state: str | None = None
+
+
+class OAuthCallbackResponse(BaseModel):
+    access_token: str
+    refresh_token: str
+    token_type: str = "bearer"
+    is_new_user: bool