fix(diagnostics): deep-review round 4 — a claim only its owner may write, and a player that keeps the Tab

Round 4 was targeted at the surfaces rounds 2 and 3 changed. 29 findings raised,
26 survived deduplication, 12 survived three adversarial readings each.

`set_delivery` renewed the claim but never checked it was still ours, while its
sibling `release_send` always did. `expire_on_commit` is off, so the token it
re-stamped came from the caller's own memory, not the row: a send that outlived
its 600 s lease came back, overwrote the take-over send's committed links with a
full-column write, and put its own dead token back. It now takes the token, reads
the row's claim with a targeted scalar (not a refresh, which would discard the
`issue_url` assigned one line earlier) and raises `SendClaimLost` rather than
writing. An issue created by a send that had already lost its claim is logged with
its number — that record is the only one that will exist.

The tracker's settings are read above the pre-call checkpoint instead of inside
`_client`. Reading them there was a SELECT one line after the commit that was
meant to end the transaction, and held it — idle, snapshot pinned — across the
repository check and the create. The comment and the module docstring said
otherwise; measuring it in pg_stat_activity is what settled it. Two more comments
were saying things the code had stopped doing: the `**state` spread does not carry
the claim (`set_delivery` strips and re-stamps it), and react-query does not
debounce the preview — the editor being unmounted is what paces it.

The composer stopped stranding the focus on `<body>`: a send that creates the
issue and then fails on an attachment unmounts the button that was just pressed,
and only the success path handed the focus on. Export no longer saves into a
refusal it does not need — the zip is the way out of a stuck send, and its
pre-save was the one step that could not survive one. The delivery banner
announces through a role instead of an `aria-live` created with its own message.

`historyState` returns a union, so `chipTone` matching on the same keys is checked
by tsc rather than by hope.

PlexPlayer was the only full-screen overlay in the tree with no focus containment
at all — Tab walked out of an opaque player into a page nobody can see. It now
carries the shared trap. Closing that hole exposed the one underneath it:
`useDismiss` registered as a layer that traps the Tab while installing no trap,
so the player stood its own down for its own menus. Measured in a real browser:
30 Tabs with the settings panel open, 9 escapes into the nav rail, and 0 after.
Escape ordering is unchanged — the panel first, then the player.

Three tests that did not hold their line: the advisory lock's call site (deleting
it left the suite green), the trail assertion whose `all()` was vacuous over an
empty filter, and a `layerStack` assertion the stand-in dialog already forced.
All eleven executable fixes mutation-checked red for the right reason; the two
comment corrections have nothing to pin.

backend/app/routes/debug_sessions.py

@@ -195,7 +195,11 @@ _new_issues = RateLimiter(max_events=5, window_seconds=3600)
 # refused for good (the tracker's own limits) never leaves `pending`. No hand reaches it.
 _sends = RateLimiter(max_events=60, window_seconds=3600)
 # The composer's live preview, rendered BY the tracker: one authenticated call each, with our token.
-# react-query debounces it to a handful a minute; this bounds what a loop can aim at the tracker.
+# The composer's toggle fires it once per switch INTO preview — the editor is the other branch, so
+# no keystroke can change the query key while the preview is up — and react-query's cache makes an
+# unchanged draft free. (It does NOT debounce, whatever this comment used to say: a key that changes
+# per keystroke is one fetch per keystroke, measured. Showing the preview BESIDE the fields would
+# need a real debounce first.) This bounds what a loop can aim at the tracker.
 _previews = RateLimiter(max_events=60, window_seconds=60)
 
 
@@ -238,10 +242,16 @@ def send_issue(
         gitea.deliver(
             db,
             session,
+            token=token,
             checkpoint=db.commit,
             before_create=count_new_issue,
             on_created=lambda: _record_created(db, user, session),
         )
+    except debugsession.SendClaimLost:
+        # This send outlived its lease and another one took the session over while it was still
+        # talking to the tracker. It stopped rather than writing over that one's progress — and a
+        # send IS under way, just not ours, which is what the caller needs to hear.
+        raise refused(409, "issue_sending") from None
     except gitea.GiteaError as exc:
         # Keep what did happen: an issue created before an upload failed stays linked, and the
         # retry does only what is still missing.