Align backend workbench API with prototype

docs/superpowers/specs/2026-07-19-backend-workbench-alignment-design.md

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+# Backend Workbench Alignment Design
+
+## Goal
+
+Align the Go backend with the latest Svelte workbench prototype. The frontend currently models a Discord-like project workbench with dynamic projects, project-scoped channels, and channel-specific first-screen data.
+
+## API Contract
+
+Add `GET /api/projects/:id/workspace`.
+
+The response intentionally matches `apps/web/src/features/workbench/types.ts` and uses camelCase field names:
+
+- `project`
+- `channels`
+- `tags`
+- `recentSessions`
+- `inbox`
+- `tasks`
+- `aiSessions`
+- `notesSources`
+- `cronPlans`
+
+## Project
+
+The project payload contains:
+
+- `id`
+- `name`
+- `description`
+- `initials`
+- `unreadCount`
+
+`unreadCount` is the count of open inbox items in the requested project.
+
+## Channels
+
+System channels are derived by the backend for every project:
+
+- `overview`
+- `inbox`
+- `tasks`
+- `ai_sessions`
+- `notes_sources`
+- `cron`
+
+Custom channels are stored in the database as project-scoped URL shortcuts:
+
+- `title`
+- `icon`
+- `url`
+- `sortOrder`
+
+System channel counts are derived from project data. Custom channels do not have counts.
+
+## Channel Data
+
+The workspace response includes first-screen channel data:
+
+- Inbox items map to mail-like message rows.
+- Tasks map to todo-style cards with a `completed` boolean.
+- AI sessions map to session list rows.
+- Notes and sources are merged into one attachment/file-management list.
+- Cron plans are metadata-only scheduled plans.
+
+Cron plans do not execute autonomous agent work in this MVP.
+
+## Access Control
+
+The endpoint only returns a workspace when the authenticated user owns the project. This follows the existing protected router and project dashboard behavior.
+
+## Tests
+
+Backend tests should verify:
+
+- The workspace rejects projects owned by another user.
+- The workspace returns camelCase fields aligned with the frontend type.
+- System channel counts are scoped to the requested project.
+- Custom channels and Cron plans are project-scoped.
+