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>

requirements.txt

@@ -8,6 +8,7 @@ openai
 psycopg2-binary
 fastapi
 uvicorn[standard]
+pydantic[email]
 httpx
 numpy
 pandas