Serve JPEG XL when a request names no format (closes #222)

A /v1/image/ URL whose last segment is a size with no format, such as
800x600 or orig, is served as JPEG XL and signed as jxl, so it shares
the signature of the same URL ending in .jxl. An encrypted URL whose
token holds no format is served as JPEG XL, as encurl.DefaultFormat is
now jxl. The generator page selects JPEG XL by default, and a form
with an empty format, or none, makes a URL whose name ends in .jxl.

The image processor no longer takes an empty format as orig: both
routes give every request a format, so it refuses a request with none
instead of keeping a second default. auto still ends with JPEG.

Model: opus-5-5

README.md

@@ -231,10 +231,11 @@ proxy in front of pixa must pass that header on unchanged. A form body over 1
 MiB is refused with 413. The image routes answer the errors listed for them with
 JSON holding `error`, `status` and `timestamp`.
 
-An image URL has this form:
+An image URL has one of these forms, the second with no format:
 
 ```
 /v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>&q=<quality>&fit=<fit>
+/v1/image/<host>/<path>/<size>?sig=<signature>&exp=<expiration>&q=<quality>&fit=<fit>
 ```
 
 Images are only fetched from origins using TLS with valid certificates, unless
@@ -245,7 +246,9 @@ A request whose query string cannot be decoded, or gives any parameter more than
 once, is refused with 400.
 
 - `<format>`: one of `orig` (or `original`), `jpeg` (or `jpg`), `png`, `webp`,
-  `avif`, `jxl` (JPEG XL), `gif`, or `auto` (below)
+  `avif`, `jxl` (JPEG XL), `gif`, or `auto` (below). A URL with no format (the
+  second form, with no dot after the size) is served as JPEG XL, the default, as
+  with `jxl`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
 - `sig` and `exp`: the signature and its expiry, needed unless the host is
   allowlisted (see Signature Specification)
@@ -321,13 +324,15 @@ nor change what it asks for.
    lasts 30 days, or until `/logout`.
 2. On the generator page, give the source image's URL, the width and height, the
    format, quality and fit, and how long the URL lasts, then submit the form
-   (`POST /generate`). Width and height both empty or `0` keep the original
-   size; if only one of them is empty or `0`, that side is scaled to keep the
-   image's proportions.
+   (`POST /generate`). The format is JPEG XL unless another is chosen; a form
+   sent with an empty format, or none, also makes a JPEG XL URL. Width and
+   height both empty or `0` keep the original size; if only one of them is empty
+   or `0`, that side is scaled to keep the image's proportions.
 3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
    it expires. `<host>` is the host the page was opened on, and the URL starts
    with `http` instead while `debug` is on. The name after the token is ignored
-   and only gives the URL a file extension, `jpg` for `orig` and `auto`.
+   and only gives the URL a file extension, `jpg` for `orig` and `auto`, and
+   `jxl` for a form with no format.
 
 The token holds the source's host, path and query and the size, format, quality,
 fit and expiry, encrypted with a key derived from `signing_key`. The source
@@ -387,8 +392,9 @@ Where:
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
 - `format` — output format, one of those listed under Routes, with `original`
-  signed as `orig` and `jpg` as `jpeg`; `auto` is signed as `auto`, not as the
-  format chosen for the request
+  signed as `orig`, `jpg` as `jpeg`, and no format as `jxl`, so a URL with no
+  format has the signature of the same URL ending in `.jxl`; `auto` is signed as
+  `auto`, not as the format chosen for the request
 - `expiration` — the URL's `exp` query parameter, the Unix timestamp when the
   signature expires; a request whose `exp` is not a whole number, an empty
   `exp=` included, is refused with 400