feat(v1.1.7-secrets): secrets SDK + table + admin API + dashboard

Encrypted per-app secrets, reachable from scripts as
secrets::{get,set,delete,list}(name) and managed from the dashboard
Secrets tab. Values are AES-256-GCM-sealed with the process master key
(picloud_shared::crypto) before they touch Postgres; the repo only ever
sees ciphertext + nonce. JSON round-trip preserves Rhai types.

- migration 0023_secrets.sql (PRIMARY KEY (app_id, name)).
- SecretsService trait (picloud-shared) + SecretsServiceImpl + repo
  (manager-core), wired into the Services bundle and Rhai engine.
- Capability::AppSecretsRead/Write (→ script:read / script:write); no
  new Scope variants (seven-scope commitment).
- Admin API GET/POST/DELETE /apps/{id}/secrets (list returns names +
  updated_at, never values).
- build_app now takes a MasterKey, sourced from PICLOUD_SECRET_KEY in
  main.rs; test callers pass a fixed test key.
- 64 KB value cap (PICLOUD_SECRET_MAX_VALUE_BYTES); no ServiceEvent
  emission (secret writes don't fire triggers, by design).

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

crates/executor-core/tests/sdk_pubsub.rs

@@ -47,6 +47,7 @@ fn make_engine(svc: Arc<RecordingPubsub>) -> Arc<Engine> {
         Arc::new(NoopHttpService),
         Arc::new(NoopFilesService),
         svc,
+        Arc::new(picloud_shared::NoopSecretsService),
     );
     Arc::new(Engine::new(Limits::default(), services))
 }