fix(api,web): separate workspace context from auth session (#534)

BetterAuth session responses contain only identity fields — workspace
context (workspaceId, currentWorkspaceId) was never returned, causing
"Workspace ID is required" on every guarded endpoint after login.

Add GET /api/workspaces endpoint (AuthGuard only, no WorkspaceGuard)
that returns user workspace memberships with auto-provisioning for
new users. Frontend auth-context now fetches workspaces after session
check and persists the default to localStorage. Race condition in
auto-provisioning is guarded by re-querying inside the transaction.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/lib/api/workspaces.ts

@@ -0,0 +1,26 @@
+/**
+ * Workspaces API Client
+ * User-scoped workspace discovery — does NOT require X-Workspace-Id header.
+ */
+
+import { apiGet } from "./client";
+
+/**
+ * A workspace entry from the user's membership list.
+ * Matches WorkspaceResponseDto from the API.
+ */
+export interface UserWorkspace {
+  id: string;
+  name: string;
+  ownerId: string;
+  role: string;
+  createdAt: string;
+}
+
+/**
+ * Fetch all workspaces the authenticated user is a member of.
+ * The API auto-provisions a default workspace if the user has none.
+ */
+export async function fetchUserWorkspaces(): Promise<UserWorkspace[]> {
+  return apiGet<UserWorkspace[]>("/api/workspaces");
+}