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/tests/threading/test_threading.py

@@ -72,6 +72,48 @@ async def test_broken_client_subject_only_joins(env: ThreadingEnv) -> None:
     assert a.thread_id == b.thread_id
 
 
+async def test_auto_replies_to_different_originals_do_not_join(env: ThreadingEnv) -> None:
+    # Two out-of-office auto-replies, same subject and same recipient, but each
+    # replying to a DIFFERENT (absent) original. They must not be subject-merged.
+    a = await env.add(
+        message_id="autoA@tvp.pl",
+        subject="Automatyczna odpowiedź: Silent Hill",
+        from_email="robert@tvp.pl",
+        to_email="me@gmail.com",
+        in_reply_to="orig1@mail.gmail.com",
+        references=["orig1@mail.gmail.com"],
+    )
+    b = await env.add(
+        message_id="autoB@wfdif.com.pl",
+        subject="Automatyczna odpowiedź: Silent Hill",
+        from_email="dyrektor@wfdif.com.pl",
+        to_email="me@gmail.com",
+        in_reply_to="orig2@mail.gmail.com",
+        references=["orig2@mail.gmail.com"],
+        minutes=5,
+    )
+    assert a.thread_id != b.thread_id
+
+
+async def test_replies_to_same_absent_parent_join(env: ThreadingEnv) -> None:
+    # Two replies that reference the SAME (absent) original — and not each other —
+    # still thread together via the shared parent reference.
+    a = await env.add(
+        message_id="r1@x.com",
+        subject="Re: Topic",
+        in_reply_to="orig@x.com",
+        references=["orig@x.com"],
+    )
+    b = await env.add(
+        message_id="r2@x.com",
+        subject="Re: Topic",
+        in_reply_to="orig@x.com",
+        references=["orig@x.com"],
+        minutes=3,
+    )
+    assert a.thread_id == b.thread_id
+
+
 async def test_distinct_topic_same_subject_does_not_join(env: ThreadingEnv) -> None:
     a = await env.add(
         message_id="a@x.com",