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/dashboard.html

@@ -1,15 +1,53 @@
 {{define "content"}}
 <div class="topbar">
   <h1>SelfPost</h1>
-  <div class="muted">
-    {{.User}} &middot;
+  <div class="actions muted">
+    <span>{{.User}}</span>
+    <form class="inline" method="post" action="/reload">
+      <button type="submit">Reload</button>
+    </form>
     <form class="inline" method="post" action="/logout">
       <button type="submit">Sign out</button>
     </form>
   </div>
 </div>
+
+{{if .Flash}}<div class="flash">{{.Flash}}</div>{{end}}
+
 <div class="card">
-  <p>You are signed in. Domains, applications and the send log arrive in the
-  next phases.</p>
+  <h2>Add a sending domain</h2>
+  <form method="post" action="/domains">
+    <label for="name">Domain</label>
+    <input id="name" name="name" type="text" placeholder="example.com"
+           autocomplete="off" autocapitalize="none" spellcheck="false"
+           value="{{.FormName}}" autofocus required>
+    {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+    <button type="submit">Add domain</button>
+  </form>
+  <p class="muted">A DKIM key is generated for the domain; you then publish the
+  shown DNS record. Adding a domain does not create an application.</p>
+</div>
+
+<div class="card">
+  <h2>Domains</h2>
+  {{if .Domains}}
+  <table>
+    <thead>
+      <tr><th>Domain</th><th>Selector</th><th>Apps</th><th></th></tr>
+    </thead>
+    <tbody>
+      {{range .Domains}}
+      <tr>
+        <td><a href="/domains/{{.ID}}">{{.Name}}</a></td>
+        <td class="muted">{{.DKIMSelector}}</td>
+        <td>{{.AppCount}}</td>
+        <td class="actions"><a class="danger" href="/domains/{{.ID}}/delete">Delete</a></td>
+      </tr>
+      {{end}}
+    </tbody>
+  </table>
+  {{else}}
+  <p class="muted">No domains yet. Add one above to get started.</p>
+  {{end}}
 </div>
 {{end}}