fix: error responses incorrectly reported success: true

[allamiro]

Fixes #280.

Problem

Four error responses carried "success": true, so API clients could interpret hard failures as success:

• db_login 'failed to store key'
• db_login 'failed to store db'
• get_user_info 'failed to retrieve session'
• _close_db 'failed to close db'

Change

All four now return "success": false. No other behavior change.

Testing

Full cargo test suite passes (9/9) in a rust:1.83 container.

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Summary by cubic

Fixed four error responses that incorrectly returned "success": true, so clients now correctly detect failures. All affected paths now return "success": false with no other behavior changes.

• Bug Fixes
  • db_login: "failed to store key"
  • db_login: "failed to store db"
  • get_user_info: "failed to retrieve session"
  • _close_db: "failed to close db"

^{Written for commit 4445988. Summary will update on new commits.}