[BE][FE] feat: Add Feishu (Lark) as native alert destination

[rusherman]

Summary

Re-opens the Feishu (Lark) native alert destination from #5775 (closed inadvertently on the maintainers' side), rebased onto the latest main and reworked to address all 7 automated review comments from the original PR.

Feishu becomes the 4th native destination alongside General / Slack / PagerDuty, delivering alerts as interactive cards to a group chat via a custom bot webhook.

Structured as 3 reviewable commits

1. refactor: extract AlertWebhookUtils (shared formatWindowDuration, formatEventType, resolveBaseUrl) and switch SlackWebhookPayloadMapper to use it — no behavior change.
2. feat: Feishu mapper/models, AlertType.FEISHU, adapter case, migration, frontend, tests.
3. chore: sync OpenAPI spec, Python/TypeScript SDKs and docs for the new enum value.

Addressing the original review

Finding	Resolution
metadata/metadataUrl NPE	resolveBaseUrl normalizes null/blank via Optional
OpenAPI/SDK not regenerated	spec + Python + TypeScript (incl. serialization) + docs updated
Broad catch (Exception)	kept as an intentional per-payload fallback, now documented
Non-reversible ENUM rollback	migration marked forward-only with a remediation note
Duplicated formatWindowDuration	extracted to AlertWebhookUtils
Duplicated formatEventType	extracted to AlertWebhookUtils
FE mapping overwrote the title prefix	Feishu uses an empty field-mapping; the title is built server-side

Testing

• Backend: FeishuWebhookPayloadMapperTest (36 cases) green; spotless:check clean
• Frontend: npm run build passes (v1 + v2)
• SDK enum values synced across Python & TypeScript (including the serialization layer)

Demo

https://www.youtube.com/watch?v=yE7wB1xJXLA