Add Feishu / Lark notification channel

[CoooderSan]

Problem

IncidentRelay's generic webhook channel sends a flat payload, while Feishu/Lark custom bots require a msg_type and content envelope. There is no native Feishu/Lark notification channel.

Change

• Add a dedicated lark notifier for Feishu and Lark custom bot webhooks.
• Support both open.feishu.cn and open.larksuite.com webhook URLs.
• Send text messages using the provider-required msg_type: text and content.text format.
• Support optional Feishu/Lark signing-secret timestamps and HMAC-SHA256 signatures.
• Validate HTTP errors and both modern and legacy provider application response formats.
• Register the channel in API schemas, OpenAPI, notifier registry, channel UI, all locale catalogs, README, and integration docs.
• Mask webhook URLs and signing secrets and preserve masked values during channel updates.
• Add backend, secret-redaction, UI asset, payload, signing, and provider-error tests.

This PR only adds native Feishu/Lark text notification support. Interactive ACK/Resolve/Shelve cards and message updates are not included.

Tests

• .venv/bin/pytest -q: 1,818 passed.
• Feishu/Lark notifier, schema, and UI tests: 20 passed.
• Ruff passed for the full app and tests trees.
• Bandit passed.
• Python compilation passed.
• JavaScript syntax validation passed for the modified script.
• Template validation passed with tests/incidentrelay.test.conf.
• Database schema validation passed.
• git diff --check passed.

Manual verification

• Verified the UI exposes a dedicated Feishu / Lark channel with webhook URL and optional signing secret fields.
• Verified edit responses mask both secrets and update placeholders restore stored values.
• Verified unsigned payloads, signed payloads, modern code/msg responses, and legacy StatusCode/StatusMessage responses in notifier tests.
• Verified live unsigned delivery through the native notifier against both open.feishu.cn and open.larksuite.com; both providers returned success.

I have read and agree to the IncidentRelay Contributor License Agreement (CLA.md).

[CoooderSan]

/sign-cla

[Aidaho12]

Hello,

Why you closed the PL? It looks good