feat: SpacetimeDB Backend Sync

[falfaddaghi]

Summary

This PR introduces the implementation plan for migrating Nexus Workflow Studio's workspace-mode persistence and real-time collaboration layer from the current filesystem + Hocuspocus/Yjs stack to SpacetimeDB. The plan covers a comprehensive 18-step migration that replaces dual-layer architecture (REST/filesystem + Hocuspocus) with a unified WebSocket-based data layer using SpacetimeDB's row-level subscriptions, server-side reducers, and automatic client-side caching. The standalone editor/localStorage mode remains fully functional and unaffected.

Implementation plan: docs/tasks/feature-spacetimedb-backend-sync-feature/plan-feature-spacetimedb-backend-sync-feature.md

ADW tracking ID: 9b6b801c

What was done

• Authored detailed implementation plan for SpacetimeDB backend sync migration
• Defined SpacetimeDB module structure with normalized table schema (workspace, workflow, brain, presence, change events)
• Specified all SpacetimeDB reducers for workspace, invite, workflow, batch ops, brain, and presence
• Planned client-side sync bridges with loop-prevention pattern mirroring _isApplyingRemote from collab-doc.ts
• Designed invite-link access control using private tables + public views filtered by ctx.sender membership
• Planned replacement of Hocuspocus in workspace mode while preserving standalone ?room= collaboration
• Defined data migration script for existing filesystem workspace/brain data
• Specified Docker deployment changes with SpacetimeDB service container
• Created E2E test specification for multi-user sync, invite flow, and reconnection scenarios
• Identified all files to create and modify across 18 implementation phases

Key changes

• New plan file: docs/tasks/feature-spacetimedb-backend-sync-feature/plan-feature-spacetimedb-backend-sync-feature.md — 368-line implementation blueprint covering SpacetimeDB module, client bridges, editor integration, access control, migration, and deployment