feat: per-script Rhai sandbox overrides with admin ceiling

Adds optional per-script overrides for the six Rhai sandbox knobs
(max_operations, max_string_size, max_array_size, max_map_size,
max_call_levels, max_expr_depth). The executor merges its defaults
with each script's overrides on every call; the manager validates
overrides against an admin-set ceiling at write time, so the
executor trusts whatever is stored.

Storage chose JSONB on the existing scripts table over six new
columns: lets future knobs land as code-only changes, keeps the
sparse common case (most scripts override nothing) cheap to store
and serialize, and matches how the manager + executor pass the
config across the wire.

  * 0002_sandbox.sql — ALTER TABLE scripts ADD COLUMN sandbox
    JSONB NOT NULL DEFAULT '{}'
  * shared::ScriptSandbox — six Option<u64> fields with
    deny_unknown_fields so typos surface as 422
  * Script.sandbox + ExecRequest.sandbox_overrides — typed end
    to end; cluster mode just serializes the same struct
  * executor-core::Limits::with_overrides — field-by-field
    replacement; tests cover the override actually tightening
    the live engine
  * manager-core::SandboxCeiling — built-in conservative
    defaults (10M ops, 1 MiB strings, 100k array/map, 128
    call/expr depth); env vars override per knob, invalid
    values warn-and-skip rather than blocking boot
  * manager-core admin API — POST/PUT accept `sandbox`; values
    above the ceiling return 422 with the specific field +
    requested + ceiling; absent or `{}` keeps platform defaults
  * picloud all-in-one — wires SandboxCeiling::from_env() into
    AdminState
  * memory_limit_mb stays in the schema, marked v1.3+ advisory
    (no enforcement until OS-level isolation lands with
    cluster-mode executors)

Verified live through Caddy:
  * /version reports schema 2, product 0.3.0
  * Script with max_operations: 500 → 507 on a 10k-iteration loop
  * Same script after PUT raising to 1M → succeeds, returns 10000
  * POST with max_operations: 1_000_000_000 → 422 (exceeds ceiling)

Tests:
  * 13 executor-core unit tests (added 2 for override semantics)
  * 20 integration tests (added 6 for sandbox CRUD + ceiling +
    unknown-field rejection + executor honoring overrides)
  * default cargo test --workspace stays green (integration tests
    remain #[ignore]'d until DATABASE_URL is set)

Bumps:
  * schema 1 → 2
  * product 0.2.0 → 0.3.0
  * SDK unchanged (scripts see nothing new)

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

crates/executor-core/tests/engine.rs

@@ -1,7 +1,7 @@
 use std::collections::BTreeMap;
 
 use picloud_executor_core::{Engine, ExecError, ExecRequest, InvocationType, Limits, LogLevel};
-use picloud_shared::{ExecutionId, RequestId, ScriptId};
+use picloud_shared::{ExecutionId, RequestId, ScriptId, ScriptSandbox};
 use serde_json::json;
 
 fn req(body: serde_json::Value) -> ExecRequest {
@@ -14,6 +14,7 @@ fn req(body: serde_json::Value) -> ExecRequest {
         path: "/test".into(),
         headers: BTreeMap::new(),
         body,
+        sandbox_overrides: ScriptSandbox::default(),
     }
 }
 
@@ -126,6 +127,40 @@ fn enforces_operation_budget() {
     assert!(matches!(err, ExecError::OperationBudgetExceeded));
 }
 
+#[test]
+fn per_request_sandbox_override_tightens_budget() {
+    // Engine default is 1M ops — the script below would finish.
+    // We override down to 500 ops on this single request; should fail.
+    let engine = engine();
+    let src = r"let n = 0; for i in 0..10000 { n += 1; } n";
+    let r = ExecRequest {
+        sandbox_overrides: ScriptSandbox {
+            max_operations: Some(500),
+            ..ScriptSandbox::default()
+        },
+        ..req(json!(null))
+    };
+    let err = engine.execute(src, r).expect_err("override should tighten");
+    assert!(matches!(err, ExecError::OperationBudgetExceeded));
+}
+
+#[test]
+fn override_only_replaces_specified_field() {
+    // Tight string size, default everything else. Strings > 32 chars
+    // should fail; loops up to default 1M ops should still pass.
+    let engine = engine();
+    let small_string_ok = r#"let s = "hello"; #{ statusCode: 200, body: s }"#;
+    let r1 = ExecRequest {
+        sandbox_overrides: ScriptSandbox {
+            max_string_size: Some(32),
+            ..ScriptSandbox::default()
+        },
+        ..req(json!(null))
+    };
+    let resp = engine.execute(small_string_ok, r1).unwrap();
+    assert_eq!(resp.body, json!("hello"));
+}
+
 #[test]
 fn runtime_error_is_mapped_to_runtime_variant() {
     let err = engine()