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/NewSession.tsx

@@ -5,7 +5,7 @@ import { useAgents, useFetch } from "../hooks";
 import { useT, type Key } from "../i18n";
 import type { ImageData } from "../types";
 import { readImage } from "./Composer";
-import { formatSize } from "./FileFigure";
+import { formatSize, imageSrc } from "./FileFigure";
 import { ModelSelect } from "./ModelSelect";
 import { takePrefill } from "../share";
 
@@ -478,7 +478,7 @@ export function NewSession({ onMenu, onCreated }: { onMenu: () => void; onCreate
                   <div className="attachments">
                     {images.map((img, i) => (
                       <div className="att" key={`i${i}`}>
-                        <img src={`data:${img.media_type};base64,${img.data}`} alt="" />
+                        <img src={imageSrc(img)} alt="" />
                         <button onClick={() => setImages((c) => c.filter((_, j) => j !== i))} aria-label={t("common.remove")}>
                           <Icon name="close" />
                         </button>