Answer 499 to an image request whose client went away (closes #339)

When the client of an image request goes away before its image is
ready, the image proxy returns context.Canceled. serveImage now logs
this once at info level, with the request_id, host and path, and
answers 499, the status nginx calls "client closed request". It was
answered 500 and logged at error level, as pixa's own failure. The
check sits in serveImage, before respondImageError, which cannot see
the request's context. A cancellation while the request's context is
live is still answered 500. README.md lists the 499 on both image
routes.

Model: opus-5-5

TODO.md

@@ -30,6 +30,16 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 on both image routes, a request whose client went away before its
+  image was ready is answered 499 and logged at info level (closes #339). It was
+  answered 500 and logged at error level, as pixa's own failure.
+    - The line is "client went away before the image was ready". It carries the
+      `request_id`, the host and the path.
+    - The 499 has no body, since nobody reads it.
+    - `serveImage` answers it before calling `respondImageError`, which cannot
+      see the request's context.
+    - An error wrapping `context.Canceled` while the request's context is live
+      is still answered 500 and logged at error level.
 - 2026-10-26 `README.md`'s Design section starts with a Concepts list (closes
   #321). Each concept gets one sentence, under the name the code gives it.
     - Storage calls the transformed images variants, in shorter sentences.