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>

internal/store/queryplan_test.go

@@ -25,12 +25,15 @@ func explain(t *testing.T, s *Store, query string, args ...any) string {
 	return strings.Join(plan, "\n")
 }
 
-// The send log is the one table that grows with the mail, and every question
-// asked of it on a path that runs per message, per log line or every few
-// minutes is answered from an index: measured with EXPLAIN QUERY PLAN against
-// the statements the store runs, not copies of them (audit 2026-10-12, M2;
-// stage 3). SQLite chooses without statistics here, as it does on a server -
-// nothing runs ANALYZE.
+// The send log is the one table that grows with the mail. The questions
+// asked of it by time or by scope - per message (the level-2 count), per log
+// line (a refusal line), every few minutes (the sweep, the notifier), per
+// page and per statistics view - are answered from an index: measured with
+// EXPLAIN QUERY PLAN against the statements the store runs, not copies of
+// them (audit 2026-10-12, M2; stage 3). Those by queue id (UpdateStatus,
+// ExpireMessage, RecoverRefusal) go by idx_send_log_queue_id and are not
+// pinned here. SQLite chooses without statistics here, as it does on a
+// server - nothing runs ANALYZE.
 func TestSendLogQueriesUseTheirIndexes(t *testing.T) {
 	s := openTestStore(t)
 	const since = "2026-10-01T00:00:00Z"