Support both v1.0 and v2.0 Azure AD token issuer formats

Azure AD issues tokens with different issuer formats depending on the
app registration's accessTokenAcceptedVersion setting:
- v1.0: https://sts.windows.net/{tenant}/
- v2.0: https://login.microsoftonline.com/{tenant}/v2.0

Update backend to accept both formats by trying each issuer in sequence.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/auth_service.py

@@ -120,19 +120,35 @@ async def verify_access_token(token: str) -> Optional[dict]:
         logger.info(f"[MSAL Backend] Found matching RSA key for kid: {kid}")
 
         # Verify and decode the token
-        # For ID tokens with OpenID scopes, audience is the client ID
-        # and issuer uses the v2.0 endpoint
-        expected_issuer = f"https://login.microsoftonline.com/{settings.AZURE_TENANT_ID}/v2.0"
-        logger.info(f"[MSAL Backend] Verifying with audience: {settings.AZURE_CLIENT_ID}")
-        logger.info(f"[MSAL Backend] Verifying with issuer: {expected_issuer}")
+        # Azure AD can issue tokens with either v1.0 or v2.0 issuer format
+        # depending on the app registration's accessTokenAcceptedVersion setting
+        v1_issuer = f"https://sts.windows.net/{settings.AZURE_TENANT_ID}/"
+        v2_issuer = f"https://login.microsoftonline.com/{settings.AZURE_TENANT_ID}/v2.0"
 
-        claims = jwt.decode(
-            token,
-            rsa_key,
-            algorithms=["RS256"],
-            audience=settings.AZURE_CLIENT_ID,
-            issuer=expected_issuer,
-        )
+        logger.info(f"[MSAL Backend] Verifying with audience: {settings.AZURE_CLIENT_ID}")
+        logger.info(f"[MSAL Backend] Accepting issuers: {v1_issuer} OR {v2_issuer}")
+
+        # Try v1 issuer first (most common for /.default scope tokens)
+        claims = None
+        for issuer in [v1_issuer, v2_issuer]:
+            try:
+                claims = jwt.decode(
+                    token,
+                    rsa_key,
+                    algorithms=["RS256"],
+                    audience=settings.AZURE_CLIENT_ID,
+                    issuer=issuer,
+                )
+                logger.info(f"[MSAL Backend] Token verified with issuer: {issuer}")
+                break
+            except JWTError as e:
+                if "issuer" in str(e).lower():
+                    continue  # Try next issuer
+                raise  # Re-raise if it's a different error
+
+        if not claims:
+            logger.warning("[MSAL Backend] Token issuer doesn't match any expected format")
+            return None
 
         logger.info(f"[MSAL Backend] Token verified successfully!")
         logger.info(f"[MSAL Backend] User: {claims.get('name', 'unknown')} ({claims.get('preferred_username', 'unknown')})")