fix: add HS256 fallback for JWT verification in local dev

Local GoTrue signs JWTs with HS256, but the JWKS endpoint returns an
empty key set since there are no RSA keys. Fall back to HS256 shared
secret verification when JWKS fails, using SUPABASE_JWT_SECRET.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/app/core/auth.py

@@ -44,26 +44,36 @@ def _extract_token(request: Request) -> str | None:
     return parts[1]
 
 
-def _verify_jwt(token: str) -> dict | None:
-    """Verify JWT using JWKS public key. Returns payload or None."""
-    client = _get_jwks_client()
-    if not client:
+def _verify_jwt_hs256(token: str) -> dict | None:
+    """Verify JWT using HS256 shared secret. Returns payload or None."""
+    if not settings.supabase_jwt_secret:
         return None
     try:
-        signing_key = client.get_signing_key_from_jwt(token)
-        payload = jwt.decode(
+        return jwt.decode(
             token,
-            signing_key.key,
-            algorithms=["RS256"],
+            settings.supabase_jwt_secret,
+            algorithms=["HS256"],
             audience="authenticated",
         )
-        return payload
-    except jwt.ExpiredSignatureError:
-        return None
     except jwt.InvalidTokenError:
         return None
-    except PyJWKClientError:
-        return None
+
+
+def _verify_jwt(token: str) -> dict | None:
+    """Verify JWT using JWKS (RS256), falling back to HS256 shared secret."""
+    client = _get_jwks_client()
+    if client:
+        try:
+            signing_key = client.get_signing_key_from_jwt(token)
+            return jwt.decode(
+                token,
+                signing_key.key,
+                algorithms=["RS256"],
+                audience="authenticated",
+            )
+        except jwt.InvalidTokenError, PyJWKClientError:
+            pass
+    return _verify_jwt_hs256(token)
 
 
 def get_current_user(request: Request) -> AuthUser | None: