Check the origin's host and port; cap the size at 1 GiB (closes #61)

access_control_allow_origin now needs a host name (ASCII letters,
digits, hyphens, dots) or an IP address, and a port, when given, from 1
to 65535, read with net/url, net/netip and strconv. Two hosts, an empty
port, no host, a bad port or a non-ASCII host name abort startup, as a
* inside the value already did.

upstream_max_response_size above 1 GiB aborts startup: the image
processor reads one byte past the limit, which wrapped negative at the
largest 64-bit value, and a response is held whole in memory.
config.example.yml states the maximum.

Model: opus-5-5

TODO.md

@@ -36,9 +36,11 @@ exhaustion
   `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`.
+  `PIXA_` variable; durations are positive Go duration strings, the size a
+  whole number of bytes up to 1 GiB, the origin `*` or one scheme, host
+  name or IP address and optional port; 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,