* fix(auth): persist auto-generated JWT secret to survive restarts
When AUTH_JWT_SECRET is not set, the auto-generated secret is now
written to .deer-flow/.jwt_secret (mode 0600) and reused on subsequent
starts. This prevents session invalidation on every restart while still
allowing explicit AUTH_JWT_SECRET in .env to take precedence.
* Apply suggestions from code review
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* Apply suggestions from code review
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* fix the lint errors of backend
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