feat: migrate JWT verification from HS256 shared secret to JWKS

Replace symmetric HS256 JWT verification with asymmetric RS256 using JWKS.
Backend now fetches and caches public keys from Supabase's JWKS endpoint
instead of using a shared secret.

- Add cryptography dependency for RS256 support
- Use PyJWKClient to fetch/cache JWKS from {SUPABASE_URL}/.well-known/jwks.json
- Remove SUPABASE_JWT_SECRET from config, docker-compose, deploy workflow, .env
- Update tests to use RS256 tokens with mocked JWKS client

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/core/auth.py

@@ -3,6 +3,7 @@ from uuid import UUID
 
 import jwt
 from fastapi import Depends, HTTPException, Request, status
+from jwt import PyJWKClient, PyJWKClientError
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -11,6 +12,8 @@ from app.core.database import get_session
 from app.models.nuzlocke_run import NuzlockeRun
 from app.models.user import User
 
+_jwks_client: PyJWKClient | None = None
+
 
 @dataclass
 class AuthUser:
@@ -21,6 +24,15 @@ class AuthUser:
     role: str | None = None
 
 
+def _get_jwks_client() -> PyJWKClient | None:
+    """Get or create a cached JWKS client."""
+    global _jwks_client
+    if _jwks_client is None and settings.supabase_url:
+        jwks_url = f"{settings.supabase_url.rstrip('/')}/.well-known/jwks.json"
+        _jwks_client = PyJWKClient(jwks_url, cache_jwk_set=True, lifespan=300)
+    return _jwks_client
+
+
 def _extract_token(request: Request) -> str | None:
     """Extract Bearer token from Authorization header."""
     auth_header = request.headers.get("Authorization")
@@ -33,14 +45,16 @@ def _extract_token(request: Request) -> str | None:
 
 
 def _verify_jwt(token: str) -> dict | None:
-    """Verify JWT against Supabase JWT secret. Returns payload or None."""
-    if not settings.supabase_jwt_secret:
+    """Verify JWT using JWKS public key. Returns payload or None."""
+    client = _get_jwks_client()
+    if not client:
         return None
     try:
+        signing_key = client.get_signing_key_from_jwt(token)
         payload = jwt.decode(
             token,
-            settings.supabase_jwt_secret,
-            algorithms=["HS256"],
+            signing_key.key,
+            algorithms=["RS256"],
             audience="authenticated",
         )
         return payload
@@ -48,6 +62,8 @@ def _verify_jwt(token: str) -> dict | None:
         return None
     except jwt.InvalidTokenError:
         return None
+    except PyJWKClientError:
+        return None
 
 
 def get_current_user(request: Request) -> AuthUser | None: