Serve both image routes with one function and one error mapping (closes #316)

HandleImage parses the path and verifies the signature, and HandleImageEnc
decrypts the token. Each then calls serveImage, which chooses the format for
auto, gets the image and serves it: the headers, ETag and 304, HEAD, the copy
and the log line. respondImageError answers the image service's errors on both
routes, and handleImageError is gone. /v1/image/ now answers 504 when the
upstream sends no response headers within upstream_fetch_timeout, as README.md
documented for /v1/e/. /v1/e/ now logs these errors at error level, as
/v1/image/ does. README.md's route tables list 504 for /v1/image/. A new test
sends the same upstream fetch errors through both routes.

Model: opus-5-5

README.md

@@ -265,15 +265,15 @@ path under `/v1/` answers 200, in maintenance mode too.
   a body whose first bytes are not those of its `Content-Type`, for an image of
   the same source URL that pixa can neither serve from its cache nor make from
   the source image still in its cache; 503 when pixa is busy or in maintenance
-  mode; 500 for any other failure.
+  mode; 504 when the upstream has not sent its response headers within
+  `upstream_fetch_timeout`, but 500 when that time runs out while the image
+  itself is still arriving; 500 for any other failure.
 - `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
   (see Encrypted URLs). Needs: nothing but the URL. Answers: 200; 304 when
   `If-None-Match` matches the image's `ETag`; 400 for a token that does not
   decrypt, or that asks for a size or fit that is not valid; 410 once it has
-  expired; 504 when the upstream has not sent its response headers within
-  `upstream_fetch_timeout`, but 500 when that time runs out while the image
-  itself is still arriving; 400 for an `Accept` header that is not valid, and
-  406, 403, 415, 429, 502, 503 and 500, as for `/v1/image/`.
+  expired; 400 for an `Accept` header that is not valid, and 406, 403, 415, 429,
+  502, 503, 504 and 500, as for `/v1/image/`.
 - `GET /robots.txt` — asks every crawler to stay away (`Disallow: /`). Needs:
   nothing. Answers: 200.
 - `GET /.well-known/healthcheck.json` — JSON with `status` (`ok`), `now`,