fix: accept ES256 (ECC P-256) JWT keys alongside RS256 in backend auth

Supabase JWT key was switched to ECC P-256, but the JWKS verification
only accepted RS256. Add ES256 to the accepted algorithms list.

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

backend/src/app/core/auth.py

@@ -60,7 +60,7 @@ def _verify_jwt_hs256(token: str) -> dict | None:
 
 
 def _verify_jwt(token: str) -> dict | None:
-    """Verify JWT using JWKS (RS256), falling back to HS256 shared secret."""
+    """Verify JWT using JWKS (RS256/ES256), falling back to HS256 shared secret."""
     client = _get_jwks_client()
     if client:
         try:
@@ -68,7 +68,7 @@ def _verify_jwt(token: str) -> dict | None:
             return jwt.decode(
                 token,
                 signing_key.key,
-                algorithms=["RS256"],
+                algorithms=["RS256", "ES256"],
                 audience="authenticated",
             )
         except jwt.InvalidTokenError: