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/store/migrations/0002_sessions.sql

@@ -1,8 +1,8 @@
--- Panel login sessions (spec 7.6.6, plan B.1). Persisted so a login survives a
--- container restart or redeploy; only the SHA-256 of the session token is
--- stored, never the token itself, so a stolen database file cannot be used to
--- sign in. expires_at implements the sliding idle timeout: it is pushed
--- forward on activity rather than being fixed at creation time.
+-- Panel login sessions. Persisted so a login survives a container restart or
+-- redeploy; only the SHA-256 of the session token is stored, never the token
+-- itself, so a stolen database file cannot be used to sign in. expires_at
+-- implements the sliding idle timeout: it is pushed forward on activity rather
+-- than being fixed at creation time.
 CREATE TABLE sessions (
     token_hash TEXT PRIMARY KEY,
     username   TEXT NOT NULL,