feat(stateful-templates): materialize group email templates (M5.5)

Close the last M5 gap: a [group] may now declare an email trigger
template, materialized into a per-descendant-app row like cron/queue.

Uses the shared-group-secret model: the template resolves its
inbound_secret_ref against the GROUP's own secret store once at apply and
seals it (resolve_and_seal + insert_email_trigger_tx generalized to a
SecretOwner / ScriptOwner). Email secrets are v0/no-AAD, so the sealed
blob is portable across rows — materialize copies the bytes verbatim into
each descendant (no master key, no reseal, no new schema column, no
threading into the CRUD hooks). Each copy has its own trigger_id / inbound
webhook URL.

- apply_service/manifest: drop the two group-email rejections; a group
  email template with an unset group secret still fails apply hard.
- materialize: add "email" to MATERIALIZED_KINDS + a byte-copy detail arm.
- trigger_repo: email_inbound_target gains AND t.app_id IS NOT NULL so a
  group TEMPLATE row is never directly invocable via its own webhook URL
  (mirrors the cron/queue app_id guards).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/picloud-cli/src/manifest.rs

@@ -75,19 +75,12 @@ impl Manifest {
         // ROUTE TEMPLATES + EVENT trigger TEMPLATES (kv/docs/files/pubsub) that
         // fan out live to descendant apps. As of §11 tail M1 a group may also
         // declare `[suppress]` to decline a template it inherits from a higher
-        // ancestor, for its whole subtree. The stateful trigger kinds
-        // (cron/queue/email) stay app-only — they need per-app state/secrets →
-        // materialization.
-        // §4.5 M5: a group may declare STATEFUL trigger templates too — cron and
-        // queue materialize a per-descendant-app row (email lands with its
-        // per-app inbound-secret handling). Only the app-specific `email` kind
-        // is still rejected on a group here.
-        if m.group.is_some() && !m.triggers.email.is_empty() {
-            anyhow::bail!(
-                "a [group] email trigger template is not yet supported \
-                 (per-app inbound secret); cron/queue/event kinds are allowed"
-            );
-        }
+        // ancestor, for its whole subtree.
+        // §4.5 M5: a group may declare STATEFUL trigger templates too — cron,
+        // queue, and email materialize a per-descendant-app row. An email
+        // template resolves its `inbound_secret_ref` against the GROUP's own
+        // secret store once at apply (shared-group-secret model); the server
+        // rejects it there if the secret is unset.
         Ok(m)
     }