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>

build/logrotate-loop.sh

@@ -1,13 +1,13 @@
 #!/bin/sh
-# Periodic logrotate for /data/log/mail.log (spec 9, 10). Rotation renames the
-# file, recreates it (`create 0640 postfix selfpost`, matching a cold container
+# Periodic logrotate for /data/log/mail.log. Rotation renames the file,
+# recreates it (`create 0640 postfix selfpost`, matching a cold container
 # start), then runs `postfix reload` (the same mechanism `postfix logrotate`
 # uses): postlogd keeps writing to the renamed inode until reload, and the
-# panel's log-tailer holds its own descriptor on that inode, so nothing
-# written before the reload is lost. `create` (rather than `nocreate`) matters
-# here beyond timing: a postlogd-triggered recreate lands the file at 0600
-# owned by postfix, which the unprivileged panel process cannot read —
-# confirmed on a live container — so logrotate must be the one to create it.
+# panel's log-tailer holds its own descriptor on that inode, so nothing written
+# before the reload is lost. `create` (rather than `nocreate`) matters here
+# beyond timing: a postlogd-triggered recreate lands the file at 0600 owned by
+# postfix, which the unprivileged panel process cannot read — confirmed on a
+# live container — so logrotate must be the one to create it.
 #
 # logrotate itself only rotates once the configured "daily" period has elapsed
 # (tracked in /var/lib/logrotate/status), so it is safe to invoke this more