🔒 [security fix] Remove hardcoded JWT secret

[davidraehles]

🎯 What: Removed the hardcoded fallback for JWT_SECRET in backend/src/middleware/auth.ts and updated README.md to use placeholders.

⚠️ Risk: A hardcoded secret allows attackers to forge JWT tokens if the JWT_SECRET environment variable is not set, potentially gaining unauthorized access to any user account.

🛡️ Solution: The application now strictly requires the JWT_SECRET environment variable to be set. If it is missing, the application will throw an informative error and fail to start, ensuring it never runs in an insecure state.

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PR created automatically by Jules for task 3533963862080789937 started by @davidraehles

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[Copilot]

Pull request overview

Removes an insecure hardcoded JWT secret fallback so the backend fails fast unless JWT_SECRET is explicitly provided, and updates documentation to use non-sensitive placeholders.

Changes:

• Require JWT_SECRET to be set at startup by throwing if missing (no hardcoded fallback).
• Update README.md env var examples to use placeholders for JWT_SECRET.
• Add internal security learning note documenting the hardcoded-secret pitfall and fix pattern.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
backend/src/middleware/auth.ts	Removes hardcoded JWT secret fallback; enforces fail-fast on missing JWT_SECRET.
README.md	Replaces JWT secret examples with placeholders.
.jules/sentinel.md	Adds documentation about hardcoded secret risks and fail-fast best practice.

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