Answer 415 for a source whose first bytes do not match its Content-Type (closes #253)

A body whose first bytes are not those of the format its Content-Type
names answered 500 on both image routes, was logged as pixa's own
error, and was fetched again on every request. Both routes now answer
415 for each error the first-bytes check returns, and the image service
logs the refused source at warning level. The failure also gets a
negative cache entry, as an upstream error status does, so the source
URL is not fetched for 5 minutes and answers 502 meanwhile. README.md
says so.

TestService_Get_ConcurrentMissesShareAFailure still expects such a
source to be fetched again and fails until its case is changed.

Model: opus-5-5

TODO.md

@@ -30,6 +30,15 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 a source whose first bytes do not match its `Content-Type` answers
+  415 (closes #253): a body whose first bytes are not those of the format its
+  `Content-Type` names, such as a JPEG sent as `image/png`, text, or a body
+  under 12 bytes, answered 500 on both image routes and was logged as an error
+  of pixa's, and each request for it fetched the whole body again. Both routes
+  now answer 415, and the image service logs it at warning level as a refused
+  source. It also gets a negative cache entry, as an upstream error status does,
+  so the source URL is not fetched again for 5 minutes and answers 502
+  meanwhile.
 - 2026-10-15 a `:` in the source path or query no longer moves where the path
   ends and the query begins (closes #247): the signed data and the cache key
   joined host, path and query with `:` and no escaping, so a URL signed for