feat(v1.1.1-kv): migrations + KvService trait + Postgres impl

First v1.1.1 commit. Adds the KV store the design notes commit to:
`(app_id, collection, key)` identity with JSONB value and a per-app
index. Trait lives in `picloud-shared` so the executor-core Rhai
bridge (next commit), the Postgres impl, and tests all depend on the
same surface without coupling crates.

The `Services` bundle grows from empty to three fields: `kv`,
`dead_letters` (NoopDeadLetterService stub — replaced by the
Postgres impl in commit 8), and `events` (NoopEventEmitter until the
outbox emitter lands with the dispatcher). Tests use
`Services::default()` for an all-noop bundle.

New capabilities `AppKvRead` / `AppKvWrite` join the Capability
enum. They map onto the existing seven-value `Scope` (script:read /
script:write) — the scope vocabulary stays locked per the
`docs/versioning.md` commitment.

Script-as-gate semantics in `KvServiceImpl`: capability check runs
when `cx.principal.is_some()`, skipped when None (public HTTP).
Cross-app isolation is enforced independently by deriving every
row's `app_id` from `cx.app_id` rather than a script-passed argument.

In-memory `KvRepo` impl + unit tests cover the round-trips, the
cross-app isolation property, empty-collection rejection,
script-as-gate behaviour for both anonymous and authed contexts,
and cursor-style pagination. Postgres impl exists; integration
testing waits for a real DB harness (see HANDBACK).

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

crates/executor-core/tests/engine.rs

@@ -27,7 +27,7 @@ fn req(body: serde_json::Value) -> ExecRequest {
 }
 
 fn engine() -> Engine {
-    Engine::new(Limits::default(), Services::new())
+    Engine::new(Limits::default(), Services::default())
 }
 
 #[test]
@@ -126,7 +126,7 @@ fn enforces_operation_budget() {
         max_operations: 1_000,
         ..Limits::default()
     };
-    let engine = Engine::new(limits, Services::new());
+    let engine = Engine::new(limits, Services::default());
     // 10_000 iterations vastly exceeds 1_000 ops.
     let src = r"let n = 0; for i in 0..10000 { n += 1; } n";
     let err = engine