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

frontend/src/components/thread/MessageBody.tsx

@@ -15,7 +15,9 @@ import { Attachments } from "./Attachments";
 export function MessageBody({ message }: { message: MessageEnvelope }) {
   const t = useT();
   const queryClient = useQueryClient();
-  const [allowed, setAllowed] = useState(false);
+  // Seed from the server's trusted-sender flag so an already-approved sender
+  // loads images and shows no "blocked" prompt on reopen.
+  const [allowed, setAllowed] = useState(message.images_allowed);
   const frameRef = useRef<HTMLIFrameElement>(null);
   const observerRef = useRef<ResizeObserver | null>(null);
   const [height, setHeight] = useState(80);
@@ -23,7 +25,10 @@ export function MessageBody({ message }: { message: MessageEnvelope }) {
   const { data, isLoading, isError } = useQuery({
     queryKey: ["message-html", message.id, allowed],
     queryFn: async () => {
-      const resp = await fetch(`/api/messages/${message.id}/html`, { credentials: "include" });
+      // ?images=1 asks the server to render remote images for this one request
+      // (the "Load images" one-off); "Always from this sender" persists separately.
+      const url = `/api/messages/${message.id}/html${allowed ? "?images=1" : ""}`;
+      const resp = await fetch(url, { credentials: "include" });
       if (!resp.ok) throw new Error("failed to load message");
       return resp.text();
     },
@@ -89,6 +94,11 @@ export function MessageBody({ message }: { message: MessageEnvelope }) {
     if (persist) {
       await api.allowRemoteImages(message.id);
       queryClient.invalidateQueries({ queryKey: ["message-html", message.id] });
+      // Refresh thread detail so the sender's images_allowed flag reflects the
+      // new trust on reopen (suppresses the prompt next time).
+      if (message.thread_id) {
+        queryClient.invalidateQueries({ queryKey: ["thread", message.thread_id] });
+      }
     }
     setAllowed(true);
   };