Users are unaware why Sentry is missing attachments

### Problem Statement

User expects some attachments on events, but nothing shows up.
Or no new attachments in the last X timeframe


### Solution Brainstorm

Turns out attachments are getting dropped because user is out of quota or attachments are too large.
But that's only found under `/stats` page.

User gets frustrated, maybe asks Sentry, only to be pointed to that page.

### Product Area

Unknown

Start at the /stats page and trace how attachment quota exhaustion and size-related drops are surfaced to users. Define the user-facing location and wording needed to explain missing attachments, then verify that both quota and oversized-attachment cases are covered.

Written by the indexing model from the issue text.