feat: Decouple chat execution from WebSocket (Phase A)

[up-guillaume]

Summary

Implement background chat execution that continues even when the browser is closed or WebSocket disconnects.

Changes

New Files

• backend/app/models/chat_execution.py — ChatExecution model for persisting chat state
• backend/app/services/chat_executor.py — Background execution service
• backend/alembic/versions/202607130808_add_chat_executions_table.py — DB migration

Modified Files

• backend/app/api/websocket.py — Modified to create background tasks instead of direct execution
• backend/app/models/__init__.py — Added ChatExecution import

Architecture

Before:

WebSocket → Execution → Response

After:

WebSocket → Create Task → Background Executor → DB
     ↑                                              ↓
     └──────────── Stream Results ←─────────────────┘

Benefits

✅ Chat execution continues when browser is closed
✅ WebSocket can reconnect and resume viewing results
✅ Execution survives network interruptions
✅ Results are persisted in database

Testing

• Unit tests for chat_executor service
• Integration test: close browser during execution
• Verify execution continues in background
• Verify results are visible after reconnect

Phase B (Future)

• Heartbeat WebSocket (ping/pong)
• Backoff exponentiel frontend
• Grace window pour reconnexion

Commits

• d8aeebb9 — feat: decouple chat execution from WebSocket
• 03a15169 — fix: set migration down_revision and resolve stash conflicts

Related

• Analysis: OpenClaw and Hermes Agent architectures
• Issue: WebSocket disconnects during long executions