fix(auth): ensure JWT sub claim is RFC 7519 compliant string

- Change TokenPayload.sub type from int to str per JWT RFC 7519
- Add user_id property to TokenPayload for int conversion
- Update token creation to serialize user_id as string
- Update token consumers to use payload.user_id
- Change dashboard port from 3000 to 8080
- Add pydantic[email] for email validation
- Update default admin email to admin@sparc.dev

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

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

docker-compose.yml

@@ -53,7 +53,7 @@ services:
     build: ./frontend
     container_name: sparc-dashboard
     ports:
-      - "3000:80"
+      - "8080:80"
     depends_on:
       - api
     restart: unless-stopped