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/shared/src/lib.rs

@@ -28,6 +28,7 @@ pub mod route;
 pub mod sandbox;
 pub mod script;
 pub mod sdk_cx;
+pub mod secrets;
 pub mod services;
 pub mod subscriber_token;
 pub mod trigger_event;
@@ -65,6 +66,10 @@ pub use route::{DispatchMode, HostKind, PathKind, Route};
 pub use sandbox::ScriptSandbox;
 pub use script::{Script, ScriptKind};
 pub use sdk_cx::SdkCallCx;
+pub use secrets::{
+    validate_secret_name, NoopSecretsService, SecretsError, SecretsListPage, SecretsService,
+    SECRET_NAME_MAX_BYTES,
+};
 pub use services::Services;
 pub use trigger_event::{
     DeadLetterEventDetail, DocsEventOp, FilesEventOp, KvEventOp, TriggerEvent,