Share one fetch and transcode among concurrent misses (closes #65)

Requests that missed the same variant at once each fetched and
transcoded it. They now share one call through
golang.org/x/sync/singleflight, keyed on the variant cache key. The
first request processes the variant with a context that does not end
with its own; the others wait for its result, holding no connection or
processing slot, and return as soon as their own context ends. The
processing request waits even then, as before. Each request counts one
miss; the processing counts its fetch and transcode once. A panic
while processing becomes an error instead of stopping pixad.

Model: opus-5-5

TODO.md

@@ -29,6 +29,17 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-09-29 share concurrent misses (closes #65): requests that miss the same
+  variant at once (the same cache key, so quality and fit included) share one
+  upstream fetch or cached source read and one transcode through
+  `golang.org/x/sync/singleflight`; the first request's processing runs with a
+  context that does not end with its own, and the others wait for its image or
+  error holding no upstream connection or processing slot, and stop waiting
+  when their own context ends; the request doing the processing waits for it
+  even then, as before; a request whose context has already ended starts
+  nothing; each request counts one miss, and the processing counts its fetch
+  and transcode once; a panic while processing becomes an error for every
+  waiting request instead of stopping pixad; documented in `README.md`.
 - 2026-09-29 maintenance mode refuses image requests (closes #71): while
   `maintenance_mode` is on, `/v1/image/` and `/v1/e/` answer 503 with a
   `Retry-After` header and the JSON error body, from one middleware in