feat(v1.1.1-dead-letters): service + Rhai SDK + admin endpoints

`PostgresDeadLetterService` lands as the real `DeadLetterService`
impl, replacing `NoopDeadLetterService` in the picloud binary's
`Services` bundle. Both methods are gated by
`Capability::AppDeadLetterManage(AppId)` — public-HTTP scripts with
`principal: None` fail the check, per design notes §4.

- `dead_letters::replay(id)` (Rhai SDK + admin endpoint): re-inserts
  the original event payload into the outbox with attempt_count=0,
  reply_to=None. The DL row is marked `resolution='replayed'`.
- `dead_letters::resolve(id, reason)` (Rhai SDK + admin endpoint):
  closes the row with `resolved_at = NOW()` and the given reason.
  CHECK constraint on the column enforces the 4-value vocabulary.
- `dead_letters::list(filter)` is intentionally NOT shipped —
  design notes §4 defers it to v1.2 to align with the eventual
  `docs::find()` query DSL.

Admin endpoints under `/api/v1/admin/apps/{id}/dead_letters/*`:
- `GET    /` (with `?unresolved=true`) → list view
- `GET    /count`                       → unresolved-count badge
- `GET    /{dl_id}`                     → row detail (full payload + error)
- `POST   /{dl_id}/replay`              → re-enqueue
- `POST   /{dl_id}/resolve` body `{reason}` → close out
All cross-app-aware: the row's `app_id` is compared against the path
param so a caller with rights on app A cannot manipulate app B's
dead letters by id alone.

The Rhai bridge for `dead_letters::*` follows the same sync↔async
pattern as the `kv::` bridge (`Handle::current().block_on(...)`
inside the spawn_blocking-wrapped Rhai engine).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/executor-core/src/sdk/mod.rs

@@ -13,6 +13,7 @@
 
 pub mod bridge;
 pub mod cx;
+pub mod dead_letters;
 pub mod kv;
 pub mod stdlib;
 
@@ -32,6 +33,5 @@ use rhai::Engine as RhaiEngine;
 /// single `<service>::register(...)` line per service.
 pub fn register_all(engine: &mut RhaiEngine, services: &Services, cx: Arc<SdkCallCx>) {
     kv::register(engine, services, cx.clone());
-    // v1.1.1 commit 8: dead_letters::register(engine, services, cx.clone());
-    let _ = cx;
+    dead_letters::register(engine, services, cx);
 }