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/api/src/workspaces/workspaces.service.ts

@@ -0,0 +1,97 @@
+import { Injectable, Logger } from "@nestjs/common";
+import { WorkspaceMemberRole } from "@prisma/client";
+import { PrismaService } from "../prisma/prisma.service";
+import type { WorkspaceResponseDto } from "./dto";
+
+@Injectable()
+export class WorkspacesService {
+  private readonly logger = new Logger(WorkspacesService.name);
+
+  constructor(private readonly prisma: PrismaService) {}
+
+  /**
+   * Get all workspaces the user is a member of.
+   *
+   * Auto-provisioning: if the user has no workspace memberships (e.g. fresh
+   * signup via BetterAuth), a default workspace is created atomically and
+   * returned.  This is the only call site for workspace bootstrapping.
+   */
+  async getUserWorkspaces(userId: string): Promise<WorkspaceResponseDto[]> {
+    const memberships = await this.prisma.workspaceMember.findMany({
+      where: { userId },
+      include: {
+        workspace: {
+          select: { id: true, name: true, ownerId: true, createdAt: true },
+        },
+      },
+      orderBy: { joinedAt: "asc" },
+    });
+
+    if (memberships.length > 0) {
+      return memberships.map((m) => ({
+        id: m.workspace.id,
+        name: m.workspace.name,
+        ownerId: m.workspace.ownerId,
+        role: m.role,
+        createdAt: m.workspace.createdAt,
+      }));
+    }
+
+    // Auto-provision a default workspace for new users.
+    // Re-query inside the transaction to guard against concurrent requests
+    // both seeing zero memberships and creating duplicate workspaces.
+    this.logger.log(`Auto-provisioning default workspace for user ${userId}`);
+
+    const workspace = await this.prisma.$transaction(async (tx) => {
+      const existing = await tx.workspaceMember.findFirst({
+        where: { userId },
+        include: {
+          workspace: {
+            select: { id: true, name: true, ownerId: true, createdAt: true },
+          },
+        },
+      });
+      if (existing) {
+        return { ...existing.workspace, alreadyExisted: true as const };
+      }
+
+      const created = await tx.workspace.create({
+        data: {
+          name: "My Workspace",
+          ownerId: userId,
+          settings: {},
+        },
+      });
+      await tx.workspaceMember.create({
+        data: {
+          workspaceId: created.id,
+          userId,
+          role: WorkspaceMemberRole.OWNER,
+        },
+      });
+      return { ...created, alreadyExisted: false as const };
+    });
+
+    if (workspace.alreadyExisted) {
+      return [
+        {
+          id: workspace.id,
+          name: workspace.name,
+          ownerId: workspace.ownerId,
+          role: WorkspaceMemberRole.OWNER,
+          createdAt: workspace.createdAt,
+        },
+      ];
+    }
+
+    return [
+      {
+        id: workspace.id,
+        name: workspace.name,
+        ownerId: workspace.ownerId,
+        role: WorkspaceMemberRole.OWNER,
+        createdAt: workspace.createdAt,
+      },
+    ];
+  }
+}