Store chat images separately; JPEG for large PNGs; gzip/brotli responses

Images of messages (pasted images, screenshots, generated images) were base64
inside the message JSON, so opening a chat downloaded all of them at once
(66 MB for a chat with 120 screenshots). They now live in a `blobs` table,
addressed by SHA-256; messages keep a reference (blob, size). The UI loads
them from /api/blobs/<hash> lazily with an immutable cache header, the runner
fills the bytes in before model requests, JSON exports stay self-contained.

- Store::add_message moves inline images out; large opaque PNGs become JPEG
  (q85) when smaller. Older chats are converted once at start-up in the
  background, unused images are removed (also after deleting a chat) and the
  database is vacuumed when that frees 32 MB or more.
- Responses are compressed with gzip/brotli (not images, SSE streams or
  binary downloads).

Example (copy of production): chat load 66 MB -> 1.5 MB (370 KB brotli),
database 90 MB -> 27 MB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/components/QueueList.tsx

@@ -3,6 +3,7 @@ import { del, post } from "../api";
 import { useT } from "../i18n";
 import { useStore } from "../store";
 import type { QueueItem } from "../types";
+import { imageSrc } from "./FileFigure";
 import { confirmDialog, promptDialog, toast } from "../dialogs";
 
 const EMPTY: QueueItem[] = [];
@@ -30,7 +31,7 @@ export function QueueList({ sessionId, busy }: { sessionId: string; busy: boolea
             {it.source === "background" && <span className="chip">{t("tool.background")}</span>}
             <span className="queue-text">{it.source === "background" ? t("queue.backgroundResult") : text || "—"}</span>
             {images.map((p, i) => (
-              <img key={i} src={`data:${p.image.media_type};base64,${p.image.data}`} alt="" />
+              <img key={i} src={imageSrc(p.image)} alt="" />
             ))}
             {it.source !== "background" && (
               <button