Fixed mentor profile links breaking when a mentor goes unavailable

A mentor toggling themselves to unavailable in Settings > Mentoring was having a bigger effect than intended: their profile page itself stopped resolving at all, returning a not-found error to anyone visiting the link, including a link someone may have saved or been sent earlier.

That's fixed. A mentor's profile page now stays up regardless of their availability. If they're not currently available, the page shows a clear message and points you toward finding another mentor, instead of looking like the mentor never existed.

We also added a lightweight check that flags (without blocking) the same message being sent to several different mentors in a short window, and published a short internal reference document defining what "approved," "available," "bookable," and a few other mentor status terms actually mean in this app today.