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/ToolCard.tsx

@@ -1,5 +1,5 @@
 import { Icon } from "./Icon";
-import { base64Size, FileFigure } from "./FileFigure";
+import { FileFigure, imageSize, imageSrc } from "./FileFigure";
 import { useT } from "../i18n";
 import { navigate } from "../router";
 import { useStore } from "../store";
@@ -166,8 +166,8 @@ export function ToolCard({ call, result, running, baseDir }: { call: ToolCall; r
           </div>
         ) : (
           result?.images?.map((img, i) => (
-            <FileFigure key={i} name={typeof call.input?.path === "string" ? call.input.path : undefined} mime={img.media_type} size={base64Size(img.data)}>
-              <img src={`data:${img.media_type};base64,${img.data}`} alt="" />
+            <FileFigure key={i} name={typeof call.input?.path === "string" ? call.input.path : undefined} mime={img.media_type} size={imageSize(img)}>
+              <img src={imageSrc(img)} alt="" loading="lazy" decoding="async" />
             </FileFigure>
           ))
         )}