sync: QRESYNC VANISHED expunge detection (SPEC §6.3 M2 fast resync)

Server-side deletions now propagate to the DB. Previously poll() detected new
messages and CONDSTORE flag deltas but never expunges (the old TODO), so mail
deleted in another client lingered as phantoms outside the small Drafts reconcile.

- provider: enable QRESYNC when advertised (_maybe_enable_qresync + qresync_enabled
  property). poll() harvests VANISHED (expunged UIDs) and changed FLAGS in one pass
  via a raw `SELECT <mbox> (QRESYNC (uidvalidity modseq))` — issued as a low-level
  SELECT because aioimaplib only surfaces untagged VANISHED lines on a sync command,
  not on an async UID FETCH. CONDSTORE-only servers (e.g. Gmail) keep the CHANGEDSINCE
  flag delta and fall back to the engine reconcile.
- parsing: parse_vanished() for `VANISHED [(EARLIER)] <uid-set>` with range expansion.
- engine: apply_delta now drops expunged rows from the search index, un-links any
  local draft mirroring a vanished UID, and publishes message.deleted for the UI.
- tests: parse_vanished unit cases + apply_delta expunge (removal, draft un-link,
  modseq advance).

Verified live: the worker issues `SELECT "INBOX" (QRESYNC (uidvalidity modseq))`
against QRESYNC-capable accounts. ruff + mypy --strict clean; 296 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

backend/tests/providers/test_parsing.py

@@ -10,6 +10,7 @@ from app.providers.imap_smtp.parsing import (
     parse_list_line,
     parse_search_uids,
     parse_select_untagged,
+    parse_vanished,
 )
 
 
@@ -57,6 +58,16 @@ def test_parse_search_uids() -> None:
     assert parse_search_uids([]) == []
 
 
+def test_parse_vanished() -> None:
+    # aioimaplib strips the leading "* " before buffering the line.
+    assert parse_vanished([b"VANISHED (EARLIER) 300:305,405"]) == [300, 301, 302, 303, 304, 305, 405]  # noqa: E501
+    # Live-expunge form (no EARLIER) and a descending range are both handled.
+    assert parse_vanished([b"VANISHED 41", b"VANISHED (EARLIER) 5:3"]) == [41, 3, 4, 5]
+    # Non-VANISHED lines are ignored; non-bytes entries are skipped.
+    assert parse_vanished([b"12 FETCH (UID 12 FLAGS (\\Seen))", b"OK done"]) == []
+    assert parse_vanished([]) == []
+
+
 def test_parse_select_untagged() -> None:
     lines = [
         b"6 EXISTS",