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>

CHANGELOG.md

@@ -3,6 +3,18 @@
 All notable changes to CodeCraft. Versions follow [Semantic Versioning](https://semver.org/);
 until 1.0, minor versions may contain breaking changes.
 
+## [Unreleased]
+
+### Changed
+
+- **Chats with many images load much faster:** images (screenshots, generated images, pasted
+  images) are stored separately instead of inside the messages and load on demand when they
+  scroll into view, cached by the browser. Large PNGs without transparency are stored as JPEG.
+  Older chats are converted once on the first start; the database usually shrinks a lot.
+  Example: a chat with 120 screenshots loads 1.5 MB instead of 66 MB.
+- **Compressed responses:** the API and the web UI are sent with gzip/brotli (not images,
+  downloads or live streams).
+
 ## [v202611-00009] – 2026-11-09
 
 ### Fixed