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>

internal/web/templates/domain_detail.html

@@ -0,0 +1,44 @@
+{{define "content"}}
+<div class="topbar">
+  <h1>{{.Domain.Name}}</h1>
+  <div class="actions muted">
+    <span>{{.User}}</span>
+    <form class="inline" method="post" action="/logout">
+      <button type="submit">Sign out</button>
+    </form>
+  </div>
+</div>
+
+<a class="back" href="/">&larr; All domains</a>
+
+<div class="card">
+  <h2>DKIM DNS record</h2>
+  <p class="muted">Publish this TXT record in the DNS for <strong>{{.Domain.Name}}</strong>.
+  It is not a secret and can be viewed at any time.</p>
+
+  <label>Host / name</label>
+  <span class="code">{{.Record.Name}}</span>
+
+  <label>Type</label>
+  <span class="code">TXT</span>
+
+  <label>Value</label>
+  <span class="code">{{.Record.Value}}</span>
+
+  <p class="muted">Also configure SPF and DMARC for the domain (see the
+  documentation). Mail is signed with selector <strong>{{.Domain.DKIMSelector}}</strong>.</p>
+</div>
+
+<div class="card">
+  <h2>Applications</h2>
+  <p class="muted">{{.Domain.AppCount}} application(s) bound to this domain.
+  Creating and managing applications arrives in the next phase.</p>
+</div>
+
+<div class="card">
+  <h2>Danger zone</h2>
+  <p class="muted">Deleting this domain also deletes its DKIM key and every
+  application bound to it.</p>
+  <a class="danger" href="/domains/{{.Domain.ID}}/delete">Delete domain</a>
+</div>
+{{end}}