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

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

backend/app/api/endpoints/oauth.py

@@ -0,0 +1,106 @@
+import secrets
+import string
+from datetime import datetime, timezone
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.admin_database import get_admin_db
+from app.core.config import settings
+from app.core.security import create_access_token, create_refresh_token
+from app.models.account import Account
+from app.models.oauth_identity import OAuthIdentity
+from app.models.user import User
+from app.schemas.oauth import OAuthCallbackPayload, OAuthCallbackResponse
+from app.services.billing import BillingService
+from app.services.oauth_providers import (
+    google_exchange_code,
+    microsoft_exchange_code,
+    OAuthProfile,
+)
+
+router = APIRouter(prefix="/auth", tags=["auth-oauth"])
+
+
+def _generate_display_code(length: int = 8) -> str:
+    """Match the helper used by /auth/register — A-Z + 0-9, length 8."""
+    alphabet = string.ascii_uppercase + string.digits
+    return "".join(secrets.choice(alphabet) for _ in range(length))
+
+
+async def _sign_in_or_register(
+    db: AsyncSession, provider: str, profile: OAuthProfile
+) -> tuple[User, bool]:
+    """Returns (user, is_new_user). Idempotent on (provider, provider_subject)."""
+    identity = (
+        await db.execute(
+            select(OAuthIdentity).where(
+                OAuthIdentity.provider == provider,
+                OAuthIdentity.provider_subject == profile.provider_subject,
+            )
+        )
+    ).scalar_one_or_none()
+
+    if identity:
+        user = (
+            await db.execute(select(User).where(User.id == identity.user_id))
+        ).scalar_one()
+        return user, False
+
+    user = (
+        await db.execute(select(User).where(User.email == profile.email))
+    ).scalar_one_or_none()
+    is_new_user = user is None
+    if is_new_user:
+        account = Account(
+            name=f"{profile.name}'s Account",
+            display_code=_generate_display_code(),
+        )
+        db.add(account)
+        await db.flush()
+        user = User(
+            email=profile.email,
+            name=profile.name,
+            password_hash=None,
+            account_id=account.id,
+            account_role="owner",
+            role="engineer",
+            email_verified_at=datetime.now(timezone.utc),
+        )
+        db.add(user)
+        await db.flush()
+        account.owner_id = user.id
+        await db.flush()
+        # start_trial commits internally; flushed account/user above.
+        await BillingService.start_trial(db, account.id)
+
+    db.add(
+        OAuthIdentity(
+            user_id=user.id,
+            provider=provider,
+            provider_subject=profile.provider_subject,
+            provider_email_at_link=profile.email,
+        )
+    )
+    await db.commit()
+    await db.refresh(user)
+    return user, is_new_user
+
+
+@router.post("/google/callback", response_model=OAuthCallbackResponse)
+async def google_callback(
+    payload: OAuthCallbackPayload,
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
+) -> OAuthCallbackResponse:
+    if not settings.GOOGLE_CLIENT_ID:
+        raise HTTPException(status_code=503, detail="Google sign-in not configured")
+    redirect_uri = f"{settings.OAUTH_REDIRECT_BASE}/auth/google/callback"
+    profile = await google_exchange_code(payload.code, redirect_uri)
+    user, is_new = await _sign_in_or_register(db, "google", profile)
+    return OAuthCallbackResponse(
+        access_token=create_access_token({"sub": str(user.id)}),
+        refresh_token=create_refresh_token({"sub": str(user.id)}),
+        is_new_user=is_new,
+    )