docs: AV9–AV12, and the two shapes that keep producing them

The register grows by the four findings from users.py, and gains the pattern
underneath several of them: a limit written on one of several equivalent paths
(three endpoints send mail, one had no limit; every list in admin.py is
bounded, the two outside it were not; the group claim was bounded at
registration and not on reload), and a bound that counts the wrong thing (a
per-IP rate limit bounds a caller, never the mailbox that receives what they
cause).

Also records that admin.py was read under the lens and needed nothing, so the
next reader does not re-derive that.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T4YmK41VsEURWFdop4EEeT

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