Fix passwordhandling

[tobiaheg]

Summary

Replaces plaintext password storage with salted scrypt hashes. New accounts and seeded users automatically receive hashed passwords, and login verifies the supplied password against the stored hash.

• Uses a unique random salt per password and timing-safe hash comparison.
• Adds npm run db-hash-passwords to convert existing plaintext passwords without changing users’ credentials. The script skips existing hashes and can safely be rerun.
• Adds migration instructions and clearer database error messages.

Testing

Four automated tests pass, covering registration, login, unique salts, incorrect passwords, and rejection of plaintext or malformed hashes.

Full TypeScript checking remains blocked by existing project configuration/type errors. Database integration tests were not run.

Before running the updated backend

Stop the backend, ensure PostgreSQL is running, and run this from the backend directory:

npm run db-hash-passwords

Restart the backend after successful conversion. Existing accounts cannot log in until their stored passwords have been converted.

Closes #80

[kfmelen]

LGTM!