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/picloud-cli/src/cmds/scripts.rs

@@ -40,10 +40,15 @@ pub async fn ls(app: Option<&str>, mode: OutputMode) -> Result<()> {
         let (apps, scripts) = tokio::try_join!(client.apps_list(), client.scripts_list_all())?;
         let slug_by_id: HashMap<AppId, String> = apps.into_iter().map(|a| (a.id, a.slug)).collect();
         for s in scripts {
-            let app_slug = slug_by_id
-                .get(&s.app_id)
-                .cloned()
-                .unwrap_or_else(|| "-".to_string());
+            // Group-owned scripts (Phase 4) have no app; show a marker rather
+            // than a slug. App-owned scripts resolve their slug as before.
+            let app_slug = match s.app_id {
+                Some(app_id) => slug_by_id
+                    .get(&app_id)
+                    .cloned()
+                    .unwrap_or_else(|| "-".to_string()),
+                None => "(group)".to_string(),
+            };
             table.row([
                 s.id.to_string(),
                 app_slug,