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>

docs/schema-migrations.md

@@ -70,13 +70,14 @@ replace the embedded set (see [Planned 2.x squash](#planned-2x-squash)).
 **Kind:** *DDL* — schema only; *data* — `INSERT`/`UPDATE` that must stay correct
 for operators upgrading from older 1.x images.
 
-Feature plans that introduced schema work: [inbound-relay](plans/inbound-relay.md),
-[domain-stats-auto-ratelimit](plans/domain-stats-auto-ratelimit.md),
-[dmarc-reports](plans/dmarc-reports.md).
+The feature plans that introduced schema work (`inbound-relay`,
+`domain-stats-auto-ratelimit`, `dmarc-reports`) were deleted once they
+shipped; what each migration was for is the table above, and the release it
+came with is in [CHANGELOG.md](../CHANGELOG.md).
 
 ---
 
-## Schema after head (v9)
+## Schema after head (v10)
 
 Tables present in a fully migrated database:
 
@@ -98,7 +99,7 @@ Tables present in a fully migrated database:
 | `dmarc_reports` | 0008 | Parsed aggregate report summaries |
 | `dmarc_report_records` | 0008 | Per-source rows inside a report |
 
-**Not present after v9:** `admin` (dropped in 0005).
+**Not present after v10:** `admin` (dropped in 0005).
 
 ---
 
@@ -174,19 +175,19 @@ git; fresh 2.x installs only run the baseline plus post-2.x files.
 Tracked as [schema-squash](roadmap.md#schema-squash). **Not** a reason to cut 2.x
 on its own — only bundled with another breaking change or an explicit major.
 
-**Goal:** embed one baseline SQL file equal to the v9 schema (plus any later 1.x
-migrations if 2.x is cut later), instead of the full 1.x chain. Fresh 2.x
-`/data` directories skip create-then-drop `admin`.
+**Goal:** embed one baseline SQL file equal to the schema at whatever the 1.x
+chain head is when the squash ships (`v10` today), instead of the full 1.x
+chain. Fresh 2.x `/data` directories skip create-then-drop `admin`.
 
 **Upgrade gate (required when squash ships):**
 
 | `user_version` | 2.x behaviour |
 |---|---|
 | `0` (empty DB) | Apply baseline; set `user_version` to new chain head |
-| `>= 9` (fully migrated 1.x) | Skip; schema already matches baseline |
-| `1`…`8` (mid-chain 1.x) | **Refuse to start** — run the last 1.x image once, then 2.x |
+| `>= N` (fully migrated 1.x) | Skip; schema already matches baseline |
+| `1`…`N-1` (mid-chain 1.x) | **Refuse to start** — run the last 1.x image once, then 2.x |
 
-Adjust the `>= N` and `1`…`N-1` thresholds to the chain head at cut time.
+`N` is the chain head at cut time (`10` today).
 
 **Done when:** baseline embedded; gate tested; [guide.md](guide.md) states that
 2.x will not open an unfinished 1.x database.