base-trixie: findings of the reviews

Security review (1c6e04b):
- The panel's database and its -wal/-shm were 0644 under a traversable
  /data, holding the panel users' password hashes and session token
  hashes. store.Open now makes them 0600 (SQLite gives the next -wal/-shm
  the database's mode: findCreateFileMode(), unixOpenSharedMemory());
  TestOpenKeepsTheDatabasePrivate.
- The entrypoint removes a symbolic link the panel user could leave where
  a /data directory, table, map, the queue or mail.log is expected, before
  its root steps chmod or create them (drop_links).
- The panel user is 10999:10999, outside the system range: in trixie
  useradd gave it group 998, the host's systemd-network on the test
  server; bookworm's 999 was the host's systemd-journal.
- security.md said the database was 0640; it was 0644.

Technical review (1c6e04b):
- mail_helpers.go: smtp-sink does add a Received: header, "(smtp-sink)"
  (smtp-sink.c:520-531); the "(Postfix)" filter leaves it out.
- CHANGELOG/roadmap: OpenDKIM's signing and From:-parsing code is
  unchanged, not all of it; the retry-policy bug predates 3.9 (any
  postconf warning); three entries moved from Fixed to Changed, as no
  released image had them.
- internal/app/sasl.go: db_dump's stdout alone is parsed.
- mynetworks comment: postinst:180, fset_all_changed.

Tests: on the server, make e2e exit 0 (712 s), go vet + go test 27
packages ok.

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

build/postfix-config.sh

@@ -387,7 +387,8 @@ postconf -e \
 # what the exceptions above and smtpd_forbid_bare_newline_exclusions (below)
 # spare. The value is the one Debian's debconf default gives (postfix
 # 3.10.13-0+deb13u1, debian/templates:124-126, "postfix/mynetworks", written
-# by debian/postfix.postinst:249-262; debian/main.cf.in:28 has 127.0.0.0/8
+# by debian/postfix.postinst:249-262 on a new main.cf, where :180,
+# fset_all_changed, marks every debconf value to be written; main.cf.in:28 has 127.0.0.0/8
 # alone), so on this image it changes nothing.
 postconf -e \
 	"mynetworks=127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128" \