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/app/providers/imap_smtp/parsing.py

@@ -135,6 +135,41 @@ def parse_fetch_lines(lines: list[bytes | bytearray]) -> list[ParsedFetch]:
     return results
 
 
+_VANISHED_RE = re.compile(rb"^VANISHED\s+(?:\(EARLIER\)\s+)?(\S+)", re.IGNORECASE)
+
+
+def _expand_uid_set(token: bytes) -> list[int]:
+    """Expand an IMAP uid-set (``1,3:5,9``) to a flat list of ints (``*`` skipped)."""
+    out: list[int] = []
+    for part in token.split(b","):
+        part = part.strip()
+        lo_b, sep, hi_b = part.partition(b":")
+        if sep:
+            if lo_b.isdigit() and hi_b.isdigit():
+                lo, hi = int(lo_b), int(hi_b)
+                out.extend(range(min(lo, hi), max(lo, hi) + 1))
+        elif part.isdigit():
+            out.append(int(part))
+    return out
+
+
+def parse_vanished(lines: list[bytes | bytearray]) -> list[int]:
+    """UIDs from QRESYNC ``VANISHED [(EARLIER)] <uid-set>`` responses (RFC 7162).
+
+    aioimaplib strips the leading ``* `` before buffering the line, so we match on
+    the bare ``VANISHED`` token. Both the SELECT-time ``(EARLIER)`` form and the
+    live-expunge form are accepted.
+    """
+    out: list[int] = []
+    for line in lines:
+        if not isinstance(line, (bytes, bytearray)):
+            continue
+        m = _VANISHED_RE.match(bytes(line).strip())
+        if m:
+            out.extend(_expand_uid_set(m.group(1)))
+    return out
+
+
 def parse_search_uids(lines: list[bytes | bytearray]) -> list[int]:
     """Parse a UID SEARCH response's first line into a list of ints."""
     if not lines: