feat(web-ui): config change webhook notifications

[SurviveM]

Requirement

When config files change, trigger webhook (Slack/Feishu/custom) with change summary, operator, and timestamp. Support configurable webhook URL and event filtering.

Verification

1. Set Feishu webhook URL in Settings -> switch provider -> Feishu receives notification "Provider switched: openai -> anthropic"
2. Edit CLAUDE.md -> webhook fires with "CLAUDE.md modified" + timestamp
3. Disable webhook -> make change -> no notification sent