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/queue_test.go

@@ -41,3 +41,34 @@ func TestParseQueueIDsOnAnEmptyQueue(t *testing.T) {
 		t.Errorf("got %v, want no ids", ids)
 	}
 }
+
+// The queue listing is held in memory and rendered into a page, so a very large
+// queue must not size the panel's allocation: the writer keeps the cap and
+// reports the rest as dropped, while still accepting every write so postqueue
+// itself is never cut short.
+func TestLimitWriterCapsOutput(t *testing.T) {
+	w := &limitWriter{remaining: 8}
+	n, err := w.Write([]byte("12345"))
+	if n != 5 || err != nil {
+		t.Fatalf("first write = %d, %v", n, err)
+	}
+	if w.truncated {
+		t.Fatal("truncated before the cap was reached")
+	}
+	n, err = w.Write([]byte("67890"))
+	if n != 5 || err != nil {
+		t.Fatalf("second write = %d, %v", n, err)
+	}
+	if !w.truncated {
+		t.Fatal("write past the cap was not recorded as truncated")
+	}
+	if got := w.buf.String(); got != "12345678" {
+		t.Fatalf("kept %q, want the first 8 bytes", got)
+	}
+	if n, err := w.Write([]byte("more")); n != 4 || err != nil {
+		t.Fatalf("write after the cap = %d, %v (must still accept)", n, err)
+	}
+	if got := w.buf.String(); got != "12345678" {
+		t.Fatalf("buffer grew past the cap: %q", got)
+	}
+}