Phase 3: sending domains + per-domain OpenDKIM signing

Add/list/delete of sending domains with per-domain DKIM keys and the
OpenDKIM tables that drive signing (spec 6, 7.2.2-4, 7.2.10).

internal/domain:
- Pure-Go RSA-2048 keygen; PKCS#1 PEM written atomically at 0640; the
  published DNS TXT record is derived from the key on disk (single source
  of truth) rather than persisted. No os/exec for key generation.
- KeyTable/SigningTable fully regenerated from the registry on every
  add/delete (idempotent), written atomically; SigningTable via refile:
  with *@domain, KeyTable with absolute key paths. Table writer refuses
  any unsafe character as a backstop (spec 7.6.4).
- Reload without root: the unprivileged panel signals OpenDKIM through
  supervisord (`supervisorctl signal USR1 opendkim`, fixed args, no
  shell, no user input — spec 7.6.3). An existing key is reused, never
  overwritten, so re-adding a domain keeps its published DNS valid.
- Service orchestrates registry -> key -> table rebuild -> reload, with
  rollback of the row if a downstream step fails; delete cascades apps
  via the DB FK and removes the key + table entries.

Infra:
- Shared `selfpost` group bridges panel (writes keys) and opendkim
  (reads them); /data/opendkim is setgid so panel-created files inherit
  the group, keys are 0640, RequireSafeKeys is disabled by design.
- opendkim.conf moves from verify-only (Mode v) to signing (Mode s).
- entrypoint.sh normalises the DKIM tree on every start (ownership,
  setgid, perms, empty tables before opendkim starts) — self-healing
  after a restore.
- supervisord control socket opened to the `selfpost` group so the panel
  can request the reload.

web/store:
- Strict domain-name validation (whitelist [a-z0-9.-], DNS shape, >=2
  labels), lower-case normalisation (spec 7.6.2).
- Domain queries with application counts; delete relies on ON DELETE
  CASCADE. Dashboard lists domains + add form; domain page shows the
  DKIM record; a dedicated confirm page warns about the app cascade
  before deletion (spec 7.2.4); manual reload button (spec 7.2.12,
  OpenDKIM side; Postfix reload lands in Phase 5).
- Authenticated routes moved to a sub-mux using Go 1.22 method/wildcard
  patterns.

Tests: validateDomain, DKIM keygen/record roundtrip, table rendering +
injection-safety, key reuse, store cascade. Verified on the dev server:
gofmt/vet/test green, image builds, container e2e (add/delete a domain,
DKIM record shown, OpenDKIM reads panel keys and reloads, keys and
tables persist across a restart).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

build/opendkim.conf

@@ -1,13 +1,29 @@
-# OpenDKIM — Phase 1 minimal config.
+# OpenDKIM — per-domain signing (spec 6).
 #
-# The goal here is only that OpenDKIM starts and exposes its milter socket so
-# the Postfix start wrapper's readiness probe passes. Per-domain signing
-# (KeyTable / SigningTable, Mode s) is generated and reloaded by the panel in
-# Phase 3; until keys exist we run in verify-only mode so startup needs no keys.
+# Signing is strictly per-domain: each sending domain has its own key pair and
+# selector. The panel generates keys under /data/opendkim/keys/<domain>/ and
+# keeps KeyTable/SigningTable in sync as domains are added or removed, then asks
+# supervisord to send this process SIGUSR1 to reload the tables (spec 6.2, 6.5).
+#
+# Both table files are created (empty) by entrypoint.sh before OpenDKIM starts,
+# so the daemon comes up cleanly on a fresh /data with no domains yet and simply
+# signs nothing until the first domain is added.
 
 Syslog                  no
 UMask                   007
-Mode                    v
+Mode                    s
+
+# SigningTable uses refile: so the "*@example.com" left-hand patterns match any
+# local-part for a domain; KeyTable maps each domain to its key and selector.
+KeyTable                /data/opendkim/KeyTable
+SigningTable            refile:/data/opendkim/SigningTable
+
+# The private keys are owned by `panel` and read by `opendkim` through the shared
+# `selfpost` group, so they are deliberately group-readable (mode 0640). That is
+# safe on this single-tenant, private bind mount, but it trips OpenDKIM's default
+# key-safety check, so the check is disabled here by design.
+RequireSafeKeys         no
+
 Socket                  local:/run/opendkim/opendkim.sock
 PidFile                 /run/opendkim/opendkim.pid
 UserID                  opendkim