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/entrypoint.sh

@@ -14,4 +14,22 @@ chown panel:panel /data
 # later phase deliberately hands to another service.
 find /data -mindepth 1 -maxdepth 1 ! -user panel -exec chown -R panel:panel {} +
 
+# DKIM key tree (spec 6, 9). The panel (user `panel`) generates keys and writes
+# the OpenDKIM tables; OpenDKIM (user `opendkim`) must read them. Normalise the
+# tree on every start so it is correct whether /data is fresh, restarted, or
+# just restored from a backup:
+#   - group `selfpost` + setgid on directories so anything the panel creates
+#     inherits the shared group OpenDKIM traverses;
+#   - private keys and tables group-readable (0640);
+#   - both table files present (empty is fine) BEFORE OpenDKIM starts, so the
+#     daemon comes up cleanly with no domains yet.
+mkdir -p /data/opendkim/keys
+for t in /data/opendkim/KeyTable /data/opendkim/SigningTable; do
+	[ -e "$t" ] || : > "$t"
+done
+chown -R panel:selfpost /data/opendkim
+find /data/opendkim -type d -exec chmod 2750 {} +
+chmod 0640 /data/opendkim/KeyTable /data/opendkim/SigningTable
+find /data/opendkim/keys -type f -name '*.private' -exec chmod 0640 {} +
+
 exec /usr/bin/supervisord -c /etc/supervisor/supervisord.conf