feat(v1.1.1-gc): dead-letter + abandoned-executions retention sweepers

Two tokio tasks spawned at startup that sweep their respective
tables on a weekly cadence (design notes §3 #9 + §4 retention).
Both use `FOR UPDATE SKIP LOCKED` on the claim query so concurrent
sweepers in cluster mode (v1.3+) don't fight each other.

Defaults: 30 days for dead_letters, 7 days for abandoned_executions.
Both env-overridable via `PICLOUD_DEAD_LETTER_RETENTION_DAYS` and
`PICLOUD_ABANDONED_EXECUTIONS_RETENTION_DAYS` (loaded into
`TriggerConfig::from_env` from commit 5).

Per-tick batch cap (5_000 rows) so a sweep can't lock up the table
in a single transaction; the inner loop continues until 0 rows
affected, after which the outer tick waits for the next week.

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

crates/picloud/src/lib.rs

@@ -220,8 +220,16 @@ pub async fn build_app(pool: PgPool, auth: AuthDeps) -> anyhow::Result<Router> {
         inbox: inbox_registry,
         outbox: outbox_writer,
     };
-    // Commit 10 wires abandoned_repo into the GC sweeper.
-    let _ = &abandoned_repo;
+    // Weekly retention sweepers for dead_letters + abandoned_executions.
+    // Defaults: 30 days / 7 days (design notes §3 #9 + §4 retention).
+    picloud_manager_core::spawn_dead_letter_gc(
+        dl_repo.clone(),
+        trigger_config.dead_letter_retention_days,
+    );
+    picloud_manager_core::spawn_abandoned_gc(
+        abandoned_repo.clone(),
+        trigger_config.abandoned_retention_days,
+    );
     let triggers_state = TriggersState {
         triggers: trigger_repo,
         apps: apps_repo.clone(),