Serve a cached variant while its source URL is negative-cached (closes #245)

Get checked the negative cache before it looked for the variant on
disk, so after one failed fetch of a source every cached variant of it
answered 502 for 5 minutes. Get now serves a cached variant first and
checks the negative cache only on a miss, before processing. The 502
line under /v1/image/ in README.md and TODO.md say so.

Model: opus-5-5

TODO.md

@@ -30,6 +30,13 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 a cached variant is served while its source URL is in the negative
+  cache (closes #245): `Get` checked the negative cache before the variant
+  cache, so once a fetch of the source failed, as one does when another variant
+  misses after the source was evicted and the upstream is down, every cached
+  variant of that source answered 502 for 5 minutes. `Get` now serves a cached
+  variant first and checks the negative cache only on a miss, before processing.
+  `README.md` says so under `/v1/image/`.
 - 2026-10-15 the host of a `/v1/image/` URL is lower-cased, and one with a port
   needs a signature (closes #246): with `cdn.example.com` allowlisted,
   `CDN.Example.com` and `cdn.example.com:8443` passed the allowlist too, and