[BUG][Feishu Channel] WebSocket connection drops without auto-reconnect

## Bug Description

Feishu channel WebSocket long connection drops after running for some time (6-24 hours), but the system does not auto-reconnect, causing no response to user messages. Restarting CoPaw restores functionality.

## Impact
- All users using Feishu/Lark channel
- P0 critical bug

## Reproduction Steps
1. Start CoPaw: `copaw start`
2. Confirm Feishu channel works
3. Let service run for 6-24 hours
4. Send message to bot in Feishu
5. Expected: Bot replies
6. Actual: No response
7. Restart CoPaw restores functionality

## Root Cause

Code location: `copaw/app/channels/feishu/channel.py`

**Issue 1: No Heartbeat** - WebSocket server closes idle connections, but CoPaw has no heartbeat

**Issue 2: No Auto-Reconnect** - `start()` called only once, no reconnect logic

**Issue 3: No State Callbacks** - Missing on_close, on_error callbacks

**Issue 4: No Timeout Detection** - No last message time check

## Log Evidence

Normal: `feishu WebSocket connecting...` then message logs

After disconnect: No logs even with user messages

After restart: Recovers

## Suggested Fixes

1. Add heartbeat loop (every 30 seconds)
2. Add auto-reconnect mechanism with retry
3. Use SDK auto-reconnect if supported
4. Add connection state monitoring

## Environment
- CoPaw: 0.2.0.post1
- OS: Windows 10
- Python: 3.10

## Severity: P0 Critical Bug

## Temporary Workaround
Daily restart: `0 3 * * * copaw restart`

Report Date: 2026-03-26