Add the four settings the README documented but pixa lacked (closes #61)

access_control_allow_origin, upstream_fetch_timeout,
upstream_max_response_size and downstream_timeout were in the README
but unknown to pixa, so a config that followed it aborted startup. Each
is now a setting with its PIXA_ variable, defaulting to the value that
was fixed in the code: *, 30s, 50 MiB and 60s. Durations are positive Go
duration strings; the size is a positive whole number of bytes; the
origin is * or one scheme and host. An invalid value aborts startup
naming the key and value. downstream_timeout replaces HTTPWriteTimeout
for the server's write timeout and the per-request timeout.

Model: opus-5-5

TODO.md

@@ -30,6 +30,15 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 add the four settings `README.md` documented but pixa did not
+  have, which aborted startup as unknown keys (closes #61):
+  `access_control_allow_origin` (default `*`, the CORS origin),
+  `upstream_fetch_timeout` (default `30s`), `upstream_max_response_size`
+  (default 50 MiB) and `downstream_timeout` (default `60s`, both the
+  server's write timeout and the per-request timeout); each has a
+  `PIXA_` variable; durations are positive Go duration strings, sizes a
+  whole number of bytes; an invalid value aborts startup naming the key
+  and the value; documented in `config.example.yml` and `README.md`.
 - 2026-09-28 refuse an unparseable `exp` on `/v1/image/` and log swallowed
   cache errors (closes #72): an `exp` in the URL that is not a whole
   number, an empty `exp=` included, is a 400 naming `exp` and the value,
@@ -253,7 +262,6 @@ exhaustion
     - X-Request-ID propagation
 - P2: auto format selection (format=auto based on Accept header)
 - P2: configuration
-    - add all configuration options from README
     - YAML config file support
 - P2: operational
     - optional Sentry error reporting