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/domain/opendkim.go

@@ -10,6 +10,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/mixeme/selfpost/internal/configsafe"
 	"github.com/mixeme/selfpost/internal/supervisor"
 )
 
@@ -202,18 +203,23 @@ func renderTables(keysDir string, domains []SigningDomain) (keyTable, signingTab
 	return []byte(kt.String()), []byte(st.String()), nil
 }
 
+// opendkimForbidden is what may never appear in a KeyTable or SigningTable
+// value: whitespace and the newline that would end the line, and the colon and
+// path separators that carry meaning inside a table entry.
+const opendkimForbidden = " \t\r\n:/\\"
+
 // assertConfigSafe rejects any domain/selector value that could break out of a
 // single table line. Domains are already whitelisted to [a-z0-9.-] and selectors
 // to a similar set before they reach here (security.md); this is defence in depth
 // against a validation gap ever letting whitespace, a newline or a field
 // separator through into a config file (security.md).
 func assertConfigSafe(domainName, selector string) error {
-	for _, v := range []string{domainName, selector} {
-		if v == "" {
-			return fmt.Errorf("opendkim: empty domain or selector")
-		}
-		if strings.ContainsAny(v, " \t\r\n:/\\") {
-			return fmt.Errorf("opendkim: unsafe character in %q", v)
+	for _, f := range []struct{ what, value string }{
+		{"domain", domainName},
+		{"selector", selector},
+	} {
+		if err := configsafe.Token(f.what, f.value, opendkimForbidden); err != nil {
+			return fmt.Errorf("opendkim: %w", err)
 		}
 	}
 	return nil