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/configsafe/configsafe.go

@@ -0,0 +1,28 @@
+// Package configsafe holds the defence-in-depth check that every generated
+// configuration file shares: a value about to be written into a table line must
+// be non-empty and must not carry a character that could end the field, end the
+// line, or otherwise change what the file means (security.md).
+//
+// The forbidden set stays with the caller rather than living here, because what
+// breaks a line differs per format — a comma separates values in a Postfix map,
+// a colon separates fields in an OpenDKIM table — and the set belongs next to
+// the writer it protects, where a reviewer of that writer can see it.
+package configsafe
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Token reports whether value is safe to interpolate into a config line. what
+// names the field for the error message ("domain", "login", …) and forbidden
+// lists the characters the format cannot carry.
+func Token(what, value, forbidden string) error {
+	if value == "" {
+		return fmt.Errorf("empty %s", what)
+	}
+	if strings.ContainsAny(value, forbidden) {
+		return fmt.Errorf("unsafe character in %s %q", what, value)
+	}
+	return nil
+}

internal/configsafe/configsafe_test.go

@@ -0,0 +1,25 @@
+package configsafe
+
+import "testing"
+
+// The check is the last line of defence before a value reaches a config file,
+// so it must reject both halves of the problem: an empty value that would write
+// a malformed line, and any character from the caller's forbidden set.
+func TestToken(t *testing.T) {
+	if err := Token("domain", "example.com", " \t\r\n,\\"); err != nil {
+		t.Fatalf("clean value rejected: %v", err)
+	}
+	if err := Token("domain", "", " "); err == nil {
+		t.Fatal("empty value accepted")
+	}
+	for _, bad := range []string{"a b", "a\tb", "a\rb", "a\nb", "a,b", "a\\b"} {
+		if err := Token("domain", bad, " \t\r\n,\\"); err == nil {
+			t.Errorf("Token(%q) = nil, want error", bad)
+		}
+	}
+	// A character outside the caller's set is that format's business, not this
+	// function's: a colon is fatal in an OpenDKIM table and fine in a texthash.
+	if err := Token("domain", "a:b", " \t\r\n,\\"); err != nil {
+		t.Errorf("character outside the forbidden set rejected: %v", err)
+	}
+}