Fix #34: Replace setup redirect allowlist prefix matching with explicit allowlist
- Replace fragile startswith() matching with explicit path matching
- Split allowlist into _EXACT_ALLOWED (exact paths) and _PREFIX_ALLOWED (prefixes)
- Prefix paths MUST end with '/' to prevent matching unintended paths like /api/setup-debug
- Paths correctly matched: /api/setup, /api/health, /api/docs, /api/redoc, /api/openapi.json, /api/setup/timezone
- Paths correctly blocked: /api/setup-debug, /api/setup123, /api/jails
- Add comprehensive Setup Guard Route Policy documentation to Backend-Development.md
- Update line numbers in documentation to reflect current implementation
This prevents future route additions from accidentally bypassing the setup guard.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>