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>

deploy/nginx/docker-compose.nginx.yml

@@ -1,7 +1,7 @@
 
-# Alternative reverse-proxy: nginx (spec 10.3). PEM files land on the host
-# exactly like the Apache+certbot scenario — nginx and certbot are close
-# cousins here, both writing/reading plain PEM under /etc/letsencrypt.
+# Alternative reverse-proxy: nginx. PEM files land on the host exactly like the
+# Apache+certbot scenario — nginx and certbot are close cousins here, both
+# writing/reading plain PEM under /etc/letsencrypt.
 #
 # This is a FRAGMENT, not a full replacement for ../docker-compose.yml: it
 # adds an nginx + certbot pair and removes the panel's host port publish (nginx
@@ -29,9 +29,9 @@ services:
     volumes: !override
       - ./data:/data
       - .:/selfpost-deploy:ro
-      # Same host directory certbot below writes into — plain bind mount,
-      # no named volume, so the PEM files are as directly inspectable as in
-      # the Apache scenario (spec 10.3).
+      # Same host directory certbot below writes into — plain bind mount, no
+      # named volume, so the PEM files are as directly inspectable as in the
+      # Apache scenario.
       - ./nginx/certbot-etc/live/mail.example.com:/etc/postfix/tls:ro
 
   nginx:

deploy/nginx/nginx.conf.example

@@ -1,6 +1,6 @@
-# nginx vhost for the SelfPost panel (spec 10.3). Proxies HTTPS to the panel
-# over the compose network; mail (465/587) is published directly by the
-# selfpost container and never touches nginx.
+# nginx vhost for the SelfPost panel. Proxies HTTPS to the panel over the
+# compose network; mail (465/587) is published directly by the selfpost
+# container and never touches nginx.
 
 server {
 	listen 80;