audit 3: backup CLI, envelope budget, directory sync, docs; findings of the reviews

Steps 3-6 of stage 3 and what the two reviews found in steps 1-6, in one
commit: the review fixes sit in the same hunks.

Step 3, CLI backup (M3, M5, L1, L2; L4 of the stage's list):
- backup.snapshotDB refuses a database that is not there and opens the
  source mode=ro (measured: mode=ro creates no file).
- selfpost-backup holds an encrypting password to
  validate.MinSecretFilePasswordLen; -o of both directions writes a
  temporary file beside the name and renames it whole; -decrypt peeks the
  first chunk (password and type authenticated) before the output exists.
- what a backup leaves out (a link under data/, a link under certs/ to a
  directory, a socket...) is told to OnWarn; certs/ with no file is
  backup.ErrNoCerts, refused by Check and Create, named by the panel.
- parseTrustedProxies: a bare address takes /128 when written with a
  colon (::ffff:10.0.0.1/32 was ::/32).
Step 4, secretfile (M4, L3): scrypt's V at most 128 MiB, r <= 64, p <= 2;
bytes after the last chunk are ErrCorrupt.
Step 5 (L7): atomicfile.SyncDir after the rename, used by Write,
replaceSASLDB and the CLI; atomicfile_test.go.
Step 6: security.md and guide on backup and restore.

Reviews (technical, security):
- H1: a restore of a server with DMARC on was refused on every start - the
  archive records no owner, tar as root left the DMARC maps root's, and the
  entrypoint only chowned the maps it named. It now hands every file beside
  queue/ in /data/postfix to panel:selfpost 0640; e2e writes an extra map
  before the backup and finds it panel:selfpost 640 after a root unpack.
- the comparison reads regular files only (Lstat, size, LimitReader); a
  record unpacked again wins over the earlier check's mark; ConsumeRestore
  removes both; the mark is synced; a journal file beside a freshly unpacked
  database is refused.
- AttachRefusalLine refuses an empty line as the old statement did; skipped
  entries named in words; -decrypt of an empty payload is no failure; guide
  wording on the shell's '>' and a killed CLI; the in-place restore's
  'mv .' (older than this stage) fixed.

Tested on the test server: go vet + go test of 25 packages, make e2e (all
pass, 643 s) on this tree.

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

build/entrypoint.sh

@@ -114,9 +114,16 @@ mkdir -p /data/postfix/queue
 # Empty files keep postfix check happy if the flag is on before any domain exists.
 for f in relay_domains transport relay_recipients tls_policy; do
 	[ -e "/data/postfix/$f" ] || : > "/data/postfix/$f"
-	chown panel:selfpost "/data/postfix/$f"
-	chmod 0640 "/data/postfix/$f"
 done
+# Every map here is the panel's to write and Postfix's to read - the sender
+# map, the inbound and the DMARC maps alike - so each file beside queue/ is
+# handed to the panel, group selfpost, whatever its name. A restored archive's
+# files arrive owned by whoever unpacked them (a backup records no owner, and
+# tar run as root makes them root's), and the first start after a restore
+# reads every one of them to compare it with the archive's record: one left
+# to root alone is a restore refused on every start (review of audit stage 3,
+# H1 - the DMARC maps were not among the names listed here).
+find /data/postfix -mindepth 1 -maxdepth 1 -type f -exec chown panel:selfpost {} + -exec chmod 0640 {} +
 chown panel:selfpost /data/postfix
 chmod 2750 /data/postfix
 
@@ -128,8 +135,6 @@ chmod 2750 /data/postfix
 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
 
 # Delivery log (architecture.md § Log tailer). postlogd writes it as user
 # `postfix`; the panel reads it for the log-tailer and the System log page. It