feat(orchestrator-core): ExecutionGate + 503/Retry-After on overflow

Adds a single global concurrency cap on the data-plane dispatch path:

  - orchestrator-core::gate::ExecutionGate wraps tokio::Semaphore.
    Non-blocking try_acquire — no queue. PICLOUD_MAX_CONCURRENT_EXECUTIONS
    env var (default 32) sets the cap.
  - LocalExecutorClient acquires a permit before spawn_blocking; the
    permit drops with the future so the slot returns automatically.
  - On refusal, ExecError::Overloaded { retry_after_secs: 1 } surfaces
    upward. ApiError::IntoResponse already maps that to 503 with a
    Retry-After header (landed in the previous commit alongside the
    variant itself).
  - picloud binary constructs the gate once at build_app and shares it
    with LocalExecutorClient.

The cap exists so a Rhai script storm can't drain the blocking-thread
pool — pushing back hard beats letting requests pile up against a
finite worker count. Per-app / per-script caps stay deferred until a
real workload demands them.

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

crates/orchestrator-core/src/client.rs

@@ -4,6 +4,8 @@ use std::time::Duration;
 use async_trait::async_trait;
 use picloud_executor_core::{Engine, ExecError, ExecRequest, ExecResponse};
 
+use crate::gate::{AcquireError, ExecutionGate};
+
 /// Maximum wall-clock time we'll wait for a single invocation, regardless
 /// of the per-script `timeout_seconds`. Provides a hard ceiling on
 /// resource usage independent of misconfigured scripts.
@@ -30,14 +32,19 @@ pub trait ExecutorClient: Send + Sync {
 /// `executor-core::Engine::execute` is synchronous; we offload it to a
 /// blocking thread so it doesn't park a Tokio worker, and apply the
 /// wall-clock timeout here.
+///
+/// Holds an `ExecutionGate` and acquires a permit before `spawn_blocking`
+/// so a script storm can't drain the blocking-thread pool. The permit
+/// drops with the future, returning the slot.
 pub struct LocalExecutorClient {
     engine: Arc<Engine>,
+    gate: Arc<ExecutionGate>,
 }
 
 impl LocalExecutorClient {
     #[must_use]
-    pub fn new(engine: Arc<Engine>) -> Self {
-        Self { engine }
+    pub fn new(engine: Arc<Engine>, gate: Arc<ExecutionGate>) -> Self {
+        Self { engine, gate }
     }
 }
 
@@ -49,6 +56,18 @@ impl ExecutorClient for LocalExecutorClient {
         req: ExecRequest,
         timeout: Duration,
     ) -> Result<ExecResponse, ExecError> {
+        // Acquire before spending any wall-clock budget. The permit is
+        // held until this future returns; spawn_blocking inherits the
+        // gating via the captured `_permit`.
+        let _permit =
+            self.gate
+                .try_acquire()
+                .map_err(
+                    |AcquireError::Overloaded { retry_after_secs }| ExecError::Overloaded {
+                        retry_after_secs,
+                    },
+                )?;
+
         let timeout = timeout.min(HARD_TIMEOUT_CAP);
         let timeout_secs = u32::try_from(timeout.as_secs()).unwrap_or(u32::MAX);