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>

.github/workflows/release.yml

@@ -1,19 +1,19 @@
 name: release
 
-# Publishes an immutable, version-tagged image on ghcr.io (spec 10.1).
+# Publishes an immutable, version-tagged image on ghcr.io.
 # Ordinary commits and bare git tag pushes do not publish anything. A published
 # GitHub Release runs this workflow directly (same pattern as gosentry / imap-scrub).
 # You can also run it manually via workflow_dispatch with an explicit SemVer X.Y.Z.
 #
-# Native per-architecture builds (see docs/development.md), not qemu:
-# running the full Postfix/OpenDKIM stack under emulation for the e2e gate
-# below is impractically slow. Each arch builds, e2e-gates and pushes its own
-# tag on its own native runner; a merge job then combines them into the one
-# manifest tag documented in deploy/docker-compose.yml. "test, then push" (not
-# push-by-digest then test) is deliberate: it means the bytes that get tagged
-# are exactly the bytes that passed e2e. Per-arch tags are pushed only so
-# imagetools can assemble the multi-arch manifest; merge removes them from GHCR
-# so operators see a single version tag (spec 10.1).
+# Native per-architecture builds (see docs/development.md), not qemu: running
+# the full Postfix/OpenDKIM stack under emulation for the e2e gate below is
+# impractically slow. Each arch builds, e2e-gates and pushes its own tag on its
+# own native runner; a merge job then combines them into the one manifest tag
+# documented in deploy/docker-compose.yml. "test, then push" (not push-by-digest
+# then test) is deliberate: it means the bytes that get tagged are exactly the
+# bytes that passed e2e. Per-arch tags are pushed only so imagetools can
+# assemble the multi-arch manifest; merge removes them from GHCR so operators
+# see a single version tag.
 on:
   release:
     types: [published]