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

config.example.yml

@@ -74,7 +74,8 @@ upstream_connections_per_host: 20
 # Time allowed for one fetch from an upstream host (default: 30s)
 upstream_fetch_timeout: 30s
 
-# Largest upstream response accepted, in bytes (default: 52428800, 50 MiB)
+# Largest upstream response accepted, in bytes, at most 1073741824
+# (1 GiB) (default: 52428800, 50 MiB)
 upstream_max_response_size: 52428800
 
 # Time allowed for answering one client request, the upstream fetch