fix(diagnostics): deep-review round 2 — a send that claims, not one that blocks

The send no longer holds the session's row lock while it talks to the tracker:
it claims the session, gives the lock back, and checkpoints each step. A Stop
is never held behind a send, while a draft, a screenshot or a second send are
refused at once instead of queueing. The claim ages out, so a request that
died mid-send does not strand the session.

What the composer says is what happened: an attachment that went is no longer
named as one that did not (the missing body edit is its own line), and the
issue's screenshot count comes from the server when the list cannot be read,
so a failed read no longer blocks saving the draft. The zip is fetched as a
request, so its pace is a message and not a saved error page. Screenshots of a
finished report can be removed again; only one still being sent is held.

A recording's log elevation can no longer outlive it. The step prompt keeps
the keyboard (the page's keys stay quiet) and traps Tab itself. An image
pasted into a dialog opened over the composer stays that dialog's.

backend/tests/test_route_conventions.py

@@ -28,6 +28,8 @@ COMMITTING_HELPERS = {
     "commits nothing)",
     "messages.py:ensure_welcome": "the websocket push after it must announce a row the client "
     "can already fetch",
+    "debug_sessions.py:_end_send": "gives back the claim a send took, in a `finally`: it runs when "
+    "the handler is already unwinding and has no commit left to reach",
 }