feat(scripts): polymorphic script owner (app XOR group) — Phase 4 foundation

Phase 4-lite C1. Make script ownership polymorphic so a script can be owned
by a GROUP (a template inherited by descendant apps) instead of an app —
mirroring vars/secrets (0048/0049), but ON DELETE RESTRICT (code is not data).

Schema (0050): `scripts.group_id` (nullable FK→groups RESTRICT), `app_id` made
nullable, `scripts_owner_exactly_one` CHECK, and the per-app name index split
into two per-owner partial-unique indexes. Existing app-owned rows keep their
exact `(app_id, lower(name))` uniqueness.

Type: `Script.app_id` becomes `Option<AppId>`; add `Script.group_id` +
`ScriptOwner` / `is_owned_by_app()`. `NewScript` gains the same polymorphic
owner. The execution-context app (what a script runs *under*) is supplied by
the invoking route/trigger/caller, never read off the script — group scripts
have no single app.

Behavior is fully preserved for app-owned scripts (the only kind creatable
today): every isolation backstop and authz site now uses `is_owned_by_app`,
which is byte-identical for app owners and **fails closed** for group owners.
A group script therefore can't yet be run, route/trigger-bound, invoked, or
managed via the app-script API — those land in C2 (group-script creation) and
C3 (chain-membership resolution + binding). The `/execute/{id}` bypass 404s a
group script (no app context to run under).

Re-blesses expected_schema.txt; note the golden was last blessed at migration
0044, so this also captures the already-committed 0045–0049 schema.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/apply_service.rs

@@ -519,7 +519,8 @@ impl ApplyService {
                 Op::Create => {
                     let bs = bundle_scripts[&key];
                     let new = NewScript {
-                        app_id,
+                        app_id: Some(app_id),
+                        group_id: None,
                         name: bs.name.clone(),
                         description: bs.description.clone(),
                         source: bs.source.clone(),
@@ -1990,7 +1991,8 @@ mod tests {
     fn script(name: &str, source: &str) -> Script {
         Script {
             id: ScriptId::from(uuid::Uuid::new_v4()),
-            app_id: AppId::from(uuid::Uuid::nil()),
+            app_id: Some(AppId::from(uuid::Uuid::nil())),
+            group_id: None,
             name: name.to_string(),
             description: None,
             version: 1,