Refuse image requests whose Referer is on referer_blocklist (closes #90)

A new setting, referer_blocklist (PIXA_REFERER_BLOCKLIST), lists hosts
whose pages may not show pixa's images. Entries are written and matched
as allowlist_hosts are, with the same matcher. Both image routes check
the Referer before the signature, the cache and the upstream fetch, and
answer 403 with the JSON error, so a blocked request costs nothing and
is refused whether or not the image is cached. No Referer, or one that
does not parse, is served; README.md says this makes the list easy to
get around. An entry of either host list that is not a host name
(letters, digits, hyphens, underscores, dots, at most one leading dot)
or an IP address now aborts startup naming the setting and the entry.

Model: opus-5-5

configs/config.example.yml

@@ -46,6 +46,8 @@ signing_key: "CHANGE_ME_generate_with_openssl_rand_base64_32"
 
 # Hosts that don't require signatures (default: none)
 # Use "." prefix for wildcard subdomain matching (e.g., ".example.com" matches "cdn.example.com")
+# An entry that is neither a host name nor an IP address (IPv6 without
+# brackets), such as one with a port or a "*." wildcard, aborts startup.
 allowlist_hosts:
   - s3.sneak.cloud
   - static.sneak.cloud
@@ -53,6 +55,16 @@ allowlist_hosts:
   - github.com
   - user-images.githubusercontent.com
 
+# Hosts whose pages may not show pixa's images, written as for
+# allowlist_hosts. A request to /v1/image/ or /v1/e/ whose Referer header
+# names one of them is answered 403 before anything is fetched, even when
+# the image is cached. A request with no Referer, or one that does not
+# parse, is served, so a site whose pages send no Referer is not stopped.
+# The login and generator pages are not covered. (default: none)
+# referer_blocklist:
+#   - leech.example
+#   - .hotlinker.example
+
 # Additional CIDR ranges to refuse when fetching upstream, extending the
 # SSRF protection. These are added to the always-enforced built-in ranges
 # (loopback, RFC 1918 private, link-local, CGNAT, benchmark, NAT64, and