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/Composer.tsx

@@ -1,6 +1,6 @@
 import { TemplatePicker } from "./TemplatePicker";
 import { Icon } from "./Icon";
-import { formatSize } from "./FileFigure";
+import { formatSize, imageSrc } from "./FileFigure";
 import { uploadFile } from "../api";
 import { useEffect, useMemo, useRef, useState } from "react";
 import { useAgents, useFetch, useModels } from "../hooks";
@@ -257,7 +257,7 @@ export function Composer({
             )}
             {images.map((img, i) => (
               <div className="att" key={i}>
-                <img src={`data:${img.media_type};base64,${img.data}`} alt="" />
+                <img src={imageSrc(img)} alt="" />
                 <button onClick={() => setImages((cur) => cur.filter((_, j) => j !== i))} aria-label={t("common.remove")}>
                   <Icon name="close" />
                 </button>