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/manager-core/migrations/0002_sandbox.sql

@@ -0,0 +1,15 @@
+-- Per-script Rhai sandbox overrides. Empty JSONB = use platform defaults.
+-- Schema is shape-only: the manager validates field names + value ranges +
+-- against the admin-set ceiling at write time. Storing JSONB lets us add
+-- new knobs without a migration (and skip them cleanly on read).
+--
+-- Recognized fields:
+--   max_operations     u64
+--   max_string_size    u64
+--   max_array_size     u64
+--   max_map_size       u64
+--   max_call_levels    u64
+--   max_expr_depth     u64
+
+ALTER TABLE scripts
+    ADD COLUMN sandbox JSONB NOT NULL DEFAULT '{}'::jsonb;