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/inbound.go

@@ -6,6 +6,8 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+
+	"github.com/mixeme/selfpost/internal/configsafe"
 )
 
 // InboundRoute is one inbound domain's Postfix map material: the relay
@@ -115,13 +117,15 @@ func assertInboundRouteSafe(r InboundRoute) error {
 	return nil
 }
 
+// texthashForbidden is what may never appear in a texthash line: whitespace and
+// the newline that would end it, the comma that separates values, and the
+// backslash.
+const texthashForbidden = " \t\r\n,\\"
+
 // assertMapToken rejects values that could break out of a texthash line.
 func assertMapToken(v, what string) error {
-	if v == "" {
-		return fmt.Errorf("postfix: empty %s", what)
-	}
-	if strings.ContainsAny(v, " \t\r\n,\\") {
-		return fmt.Errorf("postfix: unsafe character in %s %q", what, v)
+	if err := configsafe.Token(what, v, texthashForbidden); err != nil {
+		return fmt.Errorf("postfix: %w", err)
 	}
 	return nil
 }