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>

server/src/app.rs

@@ -131,9 +131,32 @@ impl App {
         });
         crate::auth::bootstrap(&app)?;
         app.clone().start_mcp();
+        app.clone().start_image_migration();
         Ok(app)
     }
 
+    /// Move inline images of older chats to the image store, drop unused images and
+    /// shrink the database file (in the background; chats load meanwhile as before).
+    fn start_image_migration(self: Arc<Self>) {
+        tokio::task::spawn_blocking(move || {
+            match self.store.migrate_inline_images() {
+                Ok(0) => {}
+                Ok(n) => tracing::info!("moved {n} images of older chats to the image store"),
+                Err(e) => tracing::warn!("moving images to the image store: {e:#}"),
+            }
+            match self.store.gc_blobs() {
+                Ok(0) => {}
+                Ok(n) => tracing::info!("removed {n} unused images"),
+                Err(e) => tracing::warn!("image cleanup: {e:#}"),
+            }
+            match self.store.vacuum_if_worthwhile() {
+                Ok(true) => tracing::info!("database compacted"),
+                Ok(false) => {}
+                Err(e) => tracing::warn!("database compaction: {e:#}"),
+            }
+        });
+    }
+
     fn start_mcp(self: Arc<Self>) {
         let servers = self.config().mcp.clone();
         let mcp = self.mcp.clone();