fix(diagnostics): deep-review round 1 — a send that cannot race, lose or repeat itself

The send, the draft and the screenshots take the session's row lock, so an
autosave or a paste during a send waits for it and then sees it, and two sends
cannot open two issues. A save keeps the delivery's progress; a failed edit
that shows the screenshots is finished by the retry; an answer that is not the
tracker's API (a redirect, a proxy page) is refused, not a crash that loses the
issue's record. Only an attempt that opened an issue counts toward the pace.

Screenshots: deleting one after the send is refused, a user's storage is
capped across sessions, new sessions and zip exports are paced, images are
stored uncompressed in the zip. One liveness rule, one session view, one image
type table.

Logging: a session's elevation is tracked apart from the user's own, so the
browser's own requests (no header) no longer lower the floor a session needs;
Stop ends it at once.

Composer: an image pasted anywhere is taken; what is still unsaved is saved on
close, and "saved" is shown only when it is; the draft is held while it is
sent; a failed screenshot read says so instead of reading as none; focus moves
to what stays; the preview follows the text size. Pill: the step prompt and a
running Stop survive the pill moving between hosts, the prompt keeps the
dialog's Tab trap, the toast region lets clicks through, other tabs hear of a
Start or Stop, the last seconds are sent before the cap, a capture with no
frame is an error.

backend/app/issues/bundle.py

@@ -23,8 +23,6 @@ from sqlalchemy.orm import Session
 from app import debugsession, logsink
 from app.models import DebugScreenshot, DebugSession
 
-_EXT = {"image/png": "png", "image/jpeg": "jpg", "image/webp": "webp"}
-
 
 @dataclass(frozen=True)
 class IssueFile:
@@ -65,7 +63,8 @@ def files(db: Session, session: DebugSession) -> list[IssueFile]:
         .order_by(DebugScreenshot.id)
     )
     for n, shot in enumerate(shots, start=1):
-        out.append(IssueFile(f"screenshot-{n}.{_EXT.get(shot.mime, 'bin')}", shot.data, shot.mime))
+        ext = debugsession.IMAGE_TYPES[shot.mime]
+        out.append(IssueFile(f"screenshot-{n}.{ext}", shot.data, shot.mime))
     return out
 
 
@@ -73,5 +72,7 @@ def zipped(items: list[IssueFile]) -> bytes:
     buf = io.BytesIO()
     with zipfile.ZipFile(buf, "w", zipfile.ZIP_DEFLATED) as z:
         for item in items:
-            z.writestr(item.name, item.data)
+            # The images are compressed already: deflating them again costs time and wins nothing.
+            method = zipfile.ZIP_STORED if item.mime.startswith("image/") else zipfile.ZIP_DEFLATED
+            z.writestr(item.name, item.data, compress_type=method)
     return buf.getvalue()