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/config.py

@@ -20,6 +20,7 @@ class Settings(BaseSettings):
     # Supabase Auth
     supabase_url: str | None = None
     supabase_anon_key: str | None = None
+    supabase_jwt_secret: str | None = None
 
 
 settings = Settings()