inbound-antispam-panel: filter lists, written as rspamd maps

Step 5/12 of docs/plans/inbound-antispam-panel.md.

Store: AddSpamListEntry, ListSpamListEntries, DeleteSpamListEntry.
internal/inbound/lists.go validates and normalises an entry - sender
address, sender domain, client address or network (canonical text, an
IPv4-mapped address as IPv4, /0 refused) - and writes the lists as six
map files under /data/antispam (ANTISPAM_DIR), each atomically, 0644 in
a 0755 directory: the filter is another user in another container. All
six are written on every change and on every start with the relay on,
empty ones included. An entry whose maps could not be written is taken
back out of the table. The writer refuses a stored value with
whitespace, a newline or # in it.

The plan's allow.map / deny.map became one file per list and entry
type: to rspamd an IP map and a sender map are different map types.

deploy/antispam/rspamd/multimap.conf holds the rules; the fragment
mounts it and ./data/antispam into the sidecar. Deny is a prefilter
reject, allow a symbol weighing -100. Tried in a throwaway rspamd 3.11
container on the test server:
  - with allow as a prefilter accept, an address on both lists was
    accepted and an IP on both was refused - the rule that ran first
    decided; with the weight, deny won in all five mixed cases;
  - IPv4, IPv6, address (any case), domain (not its subdomains) match;
  - a rewritten file is reread in about ten seconds, no restart;
  - files that appear after rspamd started are loaded the same way.

No page edits the lists yet (step 6).

Checked on the test server, uncommitted tree: go vet ./... ok,
go test ./... ok (22 packages), make e2e ok - with the relay on
/data/antispam is 755 and holds the six maps, 644, empty.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build/entrypoint.sh

@@ -112,6 +112,15 @@ for f in relay_domains transport relay_recipients tls_policy; do
 done
 chown panel:selfpost /data/postfix
 chmod 2750 /data/postfix
+
+# Filter lists for the external spam filter (written by the panel when
+# INBOUND_RELAY_ENABLE=true, read by the antispam sidecar, which mounts this
+# directory). The sidecar runs under a user of its own in another container,
+# so the directory and its files are readable by anyone: they hold sender
+# addresses and networks, no secrets.
+mkdir -p /data/antispam
+chown panel:selfpost /data/antispam
+chmod 0755 /data/antispam
 chown panel:selfpost /data/postfix/sender_login_maps
 chmod 0640 /data/postfix/sender_login_maps