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

@@ -175,20 +175,21 @@ path under `/v1/` answers 200, in maintenance mode too.
   with the page naming a field that is not valid; 500 when the URL cannot be
   made.
 - `GET /logout` — end the login session. Needs: nothing. Answers: 303 to `/`.
-- `GET` or `HEAD` `/v1/image/<host>/<path>/<size>.<format>` — an image, fetched,
-  resized and converted (below). Needs: a signature, unless the host is
-  allowlisted (see Source Hosts). Answers: 200; 304 when `If-None-Match` matches
-  the image's `ETag`; 400 for a URL or parameter that is not valid, or for the
-  format `auto` an `Accept` header that is not valid; 406 for the format `auto`
-  when `Accept` allows none of the formats it chooses from; 401 for a missing or
-  wrong signature, a missing `exp` or an `exp` in the past; 403 when the
-  request's `Referer` names a host in `referer_blocklist`, checked before the
-  signature, the cache and the upstream fetch; 403 when the upstream host, or a
-  host it redirects to, is `localhost`, ends in `.localhost` or `.local`, or has
-  an address in a blocked network (see `blocked_networks`); 502 when the
-  upstream answered with an error status, and for 5 minutes after that for the
-  same source URL; 503 when pixa is busy or in maintenance mode; 500 for any
-  other failure.
+- `GET` or `HEAD` `/v1/image/<host>/<path>/<size>.<format>`, or
+  `/v1/image/<host>/<path>/<size>` with no format — an image, fetched, resized
+  and converted (below). Needs: a signature, unless the host is allowlisted (see
+  Source Hosts). Answers: 200; 304 when `If-None-Match` matches the image's
+  `ETag`; 400 for a URL or parameter that is not valid, or for the format `auto`
+  an `Accept` header that is not valid; 406 for the format `auto` when `Accept`
+  allows none of the formats it chooses from; 401 for a missing or wrong
+  signature, a missing `exp` or an `exp` in the past; 403 when the request's
+  `Referer` names a host in `referer_blocklist`, checked before the signature,
+  the cache and the upstream fetch; 403 when the upstream host, or a host it
+  redirects to, is `localhost`, ends in `.localhost` or `.local`, or has an
+  address in a blocked network (see `blocked_networks`); 502 when the upstream
+  answered with an error status, and for 5 minutes after that for the same
+  source URL; 503 when pixa is busy or in maintenance mode; 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
@@ -231,10 +232,11 @@ 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 +247,9 @@ 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 +325,15 @@ 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 +393,9 @@ 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