feat(scripts): GET /api/v1/scripts/count + DashboardRoute wiring

After #34 the workspace store only holds the root-level scripts plus
whatever subfolders the user has expanded. DashboardRoute's
"全部脚本"/"工作副本" counts derived from scripts.length therefore
underreport the workspace total until the user navigates to /scripts
and expands every folder.

Fix: separate count endpoint + dedicated store field, mounted
independently.

Backend — backend/src/backend/scripts.py
- New endpoint GET /api/v1/scripts/count.
- Route declared BEFORE /api/v1/scripts/{script_id}/... so FastAPI's
  declaration-order matching does not interpret "count" as a script_id.
- Returns { data: { total: number }, meta: {} }; SQL is a single
  COUNT(*) on scripts filtered by workspace_id + status='active'.

Frontend — services/api.ts + context/AuthContext.tsx
- countScripts(workspaceId) client; WorkspaceBoundApi gains the field;
  AuthContext binding forwards workspaceId.

Frontend — state/scriptWorkspaceStore.ts
- scriptCount: number | null, scriptCountLoading: boolean.
- loadScriptCount() action: idempotent (no-op while in-flight), silent
  on failure (dashboard tolerates a stale count).
- Initial state and reset() clear both fields.

Frontend — features/platform/DashboardRoute.tsx
- Subscribes to scriptCount; calls loadScriptCount() on mount.
- Falls back to scripts.length until the count resolves so the
  dashboard never blanks.

Tests — backend/tests/test_count_scripts.py (new)
- 3 unit tests: scalar result handling, NULL coercion, route callable.

Verified: pytest 55 passed (52 + 3 new); pnpm typecheck clean.

frontend/app/services/api.ts

@@ -296,6 +296,22 @@ export async function listScripts(
   return apiRequest<ScriptItem[]>(`/api/v1/scripts${query}`, {}, workspaceId);
 }
 
+export async function countScripts(
+  workspaceId: string,
+): Promise<number> {
+  // Backend route /api/v1/scripts/count must be declared BEFORE the
+  // /scripts/{script_id} route on the server side. Returns
+  // { data: { total: number } } — the dashboard's single source of
+  // truth for "total active scripts in workspace", independent of the
+  // lazy-loaded scripts[] in the workspace store.
+  const envelope = await apiRequest<{ total: number }>(
+    "/api/v1/scripts/count",
+    {},
+    workspaceId,
+  );
+  return envelope.total;
+}
+
 function initialContent(scriptType: ScriptType): string {
   if (scriptType === "python") {
     return [
@@ -1466,6 +1482,7 @@ export type WorkspaceBoundApi = {
   listScripts: (
     parentPath?: Parameters<typeof listScripts>[1],
   ) => Promise<ScriptItem[]>;
+  countScripts: () => Promise<number>;
   listResources: (
     opts?: Parameters<typeof listResources>[1],
   ) => Promise<ResourceItem[]>;