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>

cmd/panel/main.go

@@ -265,10 +265,17 @@ func parseTrustedProxies(raw string) []*net.IPNet {
 			continue
 		}
 		if !strings.Contains(cidr, "/") {
-			if ip := net.ParseIP(cidr); ip != nil && ip.To4() != nil {
-				cidr += "/32"
-			} else {
+			// The prefix length goes with how the address is written: an
+			// IPv4-mapped address written as IPv6 (::ffff:10.0.0.1) is a
+			// 128-bit address, and "/32" on it is ::/32 - every address
+			// whose first 32 bits are zero (::1 among them) trusted, and the
+			// host meant not at all (audit 2026-10-12, L4). Go 1.26
+			// net.IPNet.Contains compares an IPv4 client with the mapped
+			// /128 as with 10.0.0.1/32 (net/ip.go, networkNumberAndMask).
+			if strings.Contains(cidr, ":") {
 				cidr += "/128"
+			} else {
+				cidr += "/32"
 			}
 		}
 		_, n, err := net.ParseCIDR(cidr)