Make one image URL per source on the generator page for host case and an empty path (closes #293)

The generator page passed the source host and path into the encrypted
URL as entered, so a host spelled in another letter case, or a source
with no path against the same source with /, made separate cache keys
and fetched one upstream image twice. The page now lower-cases the
source host, as /v1/image/ does, and writes an empty source path as /,
before it builds the encrypted URL. The checks the page gained for
#288 still hold.

Model: opus-5-5

TODO.md

@@ -30,6 +30,11 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-16 the generator page lower-cases the source host and writes an empty
+  source path as `/` before it makes the encrypted URL, as `/v1/image/` does
+  (closes #293). A host written in upper and in lower case, or a source URL with
+  no path and with the path `/`, made two cache keys and two upstream requests
+  for one image.
 - 2026-10-16 the source URL a request fetches is built with `net/url` from the
   source host, path and query, not joined as text (closes #288). The source
   path, decoded, is fetched percent-encoded again, so `100%25.png` fetches