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

A new setting, referer_blocklist (PIXA_REFERER_BLOCKLIST), lists hosts
written and matched as allowlist_hosts are, with the same matcher and the
same entry check; a bad entry aborts startup naming the setting and the
entry. Both image routes check the Referer first and answer 403 with the
JSON error, so a blocked request fetches nothing and is refused whether or
not the image is cached. No Referer, or one that does not parse as a URL
with a host, is served; README.md and config.example.yml say this makes the
list easy to get around. The CIDR-list entry reader is renamed listEntries
now that host lists use it too.

Model: opus-5-5

TODO.md

@@ -27,10 +27,20 @@ The disk cache is now size-bounded with LRU eviction
 
 # Next Step
 
-P2: security: referer blacklist
+P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-04 referer blocklist (closes #90): `referer_blocklist`
+  (`PIXA_REFERER_BLOCKLIST`) lists hosts, written and matched as for
+  `allowlist_hosts` with the same matcher; an entry that is not a bare host
+  aborts startup naming the setting and the entry. Both image routes refuse a
+  request whose `Referer` names a listed host with 403 and a JSON error before
+  anything else is done for it, so it fetches nothing and is refused whether or
+  not the image is cached. A request with no `Referer`, or one that does not
+  parse as a URL with a host, is served, so the list is easily got around;
+  `README.md` and `config.example.yml` say so. It does not apply to the login
+  and generator pages.
 - 2026-10-04 `TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup`
   only passes through a periodic pass (closes #189): it slept for three
   eviction intervals before writing its file, and a startup pass still running
@@ -545,7 +555,6 @@ P2: security: referer blacklist
 # Future Steps
 
 - P2: security
-    - per-IP rate limiting on the image routes
     - per-origin rate limiting
 - P2: HTTP response handling
     - Last-Modified headers