review follow-up: close the gaps left by the phase1-6 pass

Verified the tree against docs/plans/code-review-2026-08.md and finished the
tasks that were only partly done, plus two defects the pass introduced.

- bcrypt: the setup account was still hashed at DefaultCost (10). The work
  factor now has one definition, auth.BcryptCost = 12, used everywhere.
- rate limiter: protecting blocked buckets from eviction was unbounded — an
  attacker with many source addresses could grow the map by making each one
  fail its way to blocked. Blocked buckets are now evicted past
  maxBuckets * 4, nearest expiry first, logged once per window. Tests cover
  both halves.
- postqueue: the 5s timeout left the output size unbounded; reads are now
  capped at 4 MiB and report truncation.
- spec references: the ~76 `spec N` citations CHANGELOG [0.5.0] claimed were
  gone are removed from build/, deploy/, the first two migrations and the
  release workflow, with the affected comment paragraphs re-wrapped. Comments
  only; shell/YAML/SQL syntax re-checked.
- schema-migrations.md: schema section was headed v9 with the chain at v10,
  and three links pointed at deleted plan files.
- internal/configsafe: the shared empty/forbidden-character check behind the
  OpenDKIM and Postfix config writers; each writer keeps its own forbidden set.
- dropped the parseDomainRateLimitForm/parseAppRateLimitForm pass-throughs
  left behind by the parser merge (only their own tests still called them).
- plan doc now carries a status table: what shipped, what was already in
  place, what was declined (//go:build linux for the /proc reader) and the
  open CSRF-token question.

Not done, each its own phase: typed template data structs, log/slog, DMARC
service and handler tests, error-page/corrupted-backup tests, backup re-auth.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/postfix/postfix.go

@@ -11,6 +11,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/mixeme/selfpost/internal/configsafe"
 	"github.com/mixeme/selfpost/internal/supervisor"
 )
 
@@ -123,6 +124,15 @@ func appendUnique(list []string, v string) []string {
 	return append(list, v)
 }
 
+// What may never reach a sender-map line: whitespace and the newline that would
+// end it, the comma that separates values, the colon a lookup key cannot carry,
+// and the backslash. A login additionally may not carry "@" — that would make it
+// look like an address.
+const (
+	senderMapAddressForbidden = " \t\r\n,:\\"
+	senderMapLoginForbidden   = " \t\r\n,:@\\"
+)
+
 // assertMapSafe rejects any address/login value that could break out of a single
 // map line or inject a directive. Addresses are validated to a strict whitelist
 // (letters, digits, '@', '.', '-', '_', '+') and logins to an even stricter one
@@ -130,14 +140,11 @@ func appendUnique(list []string, v string) []string {
 // letting whitespace, a newline or a comma (the value separator) through into
 // the file (security.md).
 func assertMapSafe(address, login string) error {
-	if address == "" || login == "" {
-		return fmt.Errorf("postfix: empty address or login")
+	if err := configsafe.Token("address", address, senderMapAddressForbidden); err != nil {
+		return fmt.Errorf("postfix: %w", err)
 	}
-	if strings.ContainsAny(address, " \t\r\n,:\\") {
-		return fmt.Errorf("postfix: unsafe character in address %q", address)
-	}
-	if strings.ContainsAny(login, " \t\r\n,:@\\") {
-		return fmt.Errorf("postfix: unsafe character in login %q", login)
+	if err := configsafe.Token("login", login, senderMapLoginForbidden); err != nil {
+		return fmt.Errorf("postfix: %w", err)
 	}
 	return nil
 }