M2: Meili search + saved searches, sync windows + backfill progress, threading & image fixes

Search:
- Meilisearch backend (app/search/meili.py) + factory wiring; pinned SDK 2.7
- Search-as-you-type: debounced query + Postgres prefix matching, built from
  the DB tokenizer so full email addresses match (email stays one lexeme)
- Saved searches API + sidebar virtual mailboxes; save affordance in the box

Account sync windows (SPEC §12 M2):
- Choose at setup: All (recent 30d first, then remainder), Last 3 months, New only
- Per-account sync_window + "Sync remaining mail" from settings (idempotent re-run)
- Provider windowed backfill (SINCE/BEFORE), message_count, newest_uid
- migration 0012: accounts.sync_window + backfill_total/backfill_done

Backfill progress UX:
- Floating progress card (auto-dissolves after 10s) + spinning sidebar ring that
  fills with the account colour; driven by sync.progress over WS

Thread list pagination:
- Server-side offset + accurate grand total, with filter/order in SQL; working
  prev/next across all synced mail

Reading panel:
- "Load images" one-off now actually renders images (?images=1) without trusting
  the sender; envelope carries images_allowed so a trusted sender shows no prompt

Threading:
- Subject fallback no longer merges messages that declare an explicit (different)
  parent; true siblings still group via a shared-parent reference match. Fixes
  unrelated auto-replies with the same subject landing in one thread

Backend 257 tests green, ruff + mypy --strict clean; frontend tsc/eslint/vitest clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

backend/app/search/factory.py

@@ -10,12 +10,14 @@ _backend: SearchBackend | None = None
 
 
 def get_search_backend() -> SearchBackend:
-    """Return the process-wide SearchBackend (``pg`` in M0-M1, ``meili`` in M2)."""
+    """Return the process-wide SearchBackend (``pg`` or ``meili`` per SEARCH_BACKEND)."""
     global _backend
     if _backend is None:
         which = get_settings().search_backend
         if which == "meili":
-            # TODO(spec): Meilisearch backend lands in M2.
-            raise NotImplementedError("meili backend is implemented in M2")
-        _backend = PgSearchBackend()
+            from app.search.meili import MeiliSearchBackend
+
+            _backend = MeiliSearchBackend()
+        else:
+            _backend = PgSearchBackend()
     return _backend