Implement version history and restore API endpoints

[davstur]

Overview

Build API endpoints to list version history for a topic and restore a previous version. The versions table and snapshot creation already exist (from #9, #21, #24). This adds the read + restore endpoints.

User Story

User Story: /docs/user-stories/active/version-history.md

As a user, I want to see a history of changes and restore any previous version, so I can undo AI changes I don't like.

Suggested Priority: high 🟠

Requirements

List Versions

• GET /api/topics/{topic_id}/versions returns version history
• Each version: id, action, created_at, target node label (from snapshot)
• Sorted by created_at descending (newest first)

Get Version Snapshot

• GET /api/topics/{topic_id}/versions/{version_id} returns full snapshot
• Returns the nodes array from the JSONB snapshot field

Restore Version

• POST /api/topics/{topic_id}/versions/{version_id}/restore
• Creates a new version snapshot of current state (action="restore")
• Deletes all current nodes for the topic
• Inserts nodes from the snapshot
• Returns the restored topic with nodes

Technical Approach

• Versions table exists with topic_id, snapshot (JSONB), action, created_at
• Restore flow: snapshot current → delete all nodes → insert from target snapshot → return
• Use the same FK-aware delete order as _cleanup_partial_insert
• The restored nodes need new UUIDs (can't reuse old IDs if they conflict)

Related

• User Story: /docs/user-stories/active/version-history.md
• Versions table: supabase/migrations/20260330120000_create_schema.sql
• Existing snapshots from: expand (Implement EXPAND_NODE API endpoint with version snapshots #21), create subtopics (Implement SUGGEST_SUBTOPICS and CREATE_SUBTOPICS API endpoints #24)

[davstur]

🤖 Claude Code has started implementing this issue on branch feature/29-30-version-history (combined with #30)