Give /v1/e/ ETag, 304 and HEAD as /v1/image/ has (closes #84)

The ETag and If-None-Match code of the /v1/image/ handler becomes
notModified, which both image handlers call; /v1/e/ answers HEAD with
the headers only and is routed for HEAD. HandleImageEnc was at the
80-line function limit, so its token checks move unchanged into
parseImageEncRequest, as parseImageRequest does for /v1/image/. No Vary
is added: only the image routes' CORS headers depend on a request
header, and go-chi/cors already sends Vary: Origin with them.

Model: opus-5-5

TODO.md

@@ -29,6 +29,19 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 request IDs returned and passed on, and `/v1/e/` revalidates
+  (closes #84): a middleware right after chi's `RequestID` sets `X-Request-ID`
+  on every response from the ID `RequestID` stores in the request context, which
+  is the request's own `X-Request-ID` when it sent one; the upstream fetch sends
+  that ID, and the "upstream fetched", "image converted" and "image served" log
+  lines carry it as `request_id`, a fetch shared by several requests carrying
+  the first request's; `/v1/e/` sets `ETag`, answers a matching `If-None-Match`
+  with 304 and is routed for `HEAD`, the `ETag` and 304 code being
+  `notModified`, which `/v1/image/` calls too; its token checks moved unchanged
+  into `parseImageEncRequest` to keep `HandleImageEnc` within the line limit; no
+  `Vary` is added, as no response depends on a request header except the image
+  routes' CORS headers, for which `go-chi/cors` already sends `Vary: Origin`;
+  `Vary: Accept` is left to #88.
 - 2026-10-04 routes, encrypted URLs and config file documented (closes #75):
   "Routes" in `README.md` lists every route with its method, purpose, what it
   needs and the status codes it answers with, and says `q` and `fit` are part
@@ -438,7 +451,6 @@ P2: security: referer blacklist
 - P2: HTTP response handling
     - Last-Modified headers
     - Vary header for content negotiation
-    - X-Request-ID propagation
 - P2: auto format selection (format=auto based on Accept header)
 - P2: configuration
     - YAML config file support