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

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

backend/app/services/oauth_providers.py

@@ -0,0 +1,71 @@
+"""OAuth provider helpers. Each provider exposes:
+- exchange_code(code, redirect_uri) -> OAuthProfile
+"""
+from dataclasses import dataclass
+
+import httpx
+from app.core.config import settings
+
+
+@dataclass
+class OAuthProfile:
+    provider_subject: str
+    email: str
+    name: str
+
+
+async def google_exchange_code(code: str, redirect_uri: str) -> OAuthProfile:
+    async with httpx.AsyncClient(timeout=10) as cli:
+        token_response = await cli.post(
+            "https://oauth2.googleapis.com/token",
+            data={
+                "code": code,
+                "client_id": settings.GOOGLE_CLIENT_ID,
+                "client_secret": settings.GOOGLE_CLIENT_SECRET,
+                "redirect_uri": redirect_uri,
+                "grant_type": "authorization_code",
+            },
+        )
+        token_response.raise_for_status()
+        access_token = token_response.json()["access_token"]
+
+        userinfo = await cli.get(
+            "https://openidconnect.googleapis.com/v1/userinfo",
+            headers={"Authorization": f"Bearer {access_token}"},
+        )
+        userinfo.raise_for_status()
+        data = userinfo.json()
+        return OAuthProfile(
+            provider_subject=data["sub"],
+            email=data["email"],
+            name=data.get("name") or data["email"].split("@")[0],
+        )
+
+
+async def microsoft_exchange_code(code: str, redirect_uri: str) -> OAuthProfile:
+    async with httpx.AsyncClient(timeout=10) as cli:
+        token_response = await cli.post(
+            "https://login.microsoftonline.com/common/oauth2/v2.0/token",
+            data={
+                "code": code,
+                "client_id": settings.MS_CLIENT_ID,
+                "client_secret": settings.MS_CLIENT_SECRET,
+                "redirect_uri": redirect_uri,
+                "grant_type": "authorization_code",
+                "scope": "openid email profile",
+            },
+        )
+        token_response.raise_for_status()
+        access_token = token_response.json()["access_token"]
+
+        userinfo = await cli.get(
+            "https://graph.microsoft.com/v1.0/me",
+            headers={"Authorization": f"Bearer {access_token}"},
+        )
+        userinfo.raise_for_status()
+        data = userinfo.json()
+        return OAuthProfile(
+            provider_subject=data["id"],
+            email=data.get("mail") or data["userPrincipalName"],
+            name=data.get("displayName") or data["userPrincipalName"].split("@")[0],
+        )