feat(webhooks): add Slack and Discord delivery format presets

[parth0025]

S2-01 / S2-03 (phase 1) — Slack & Discord webhook presets

Lets an outgoing webhook target a Slack or Discord incoming webhook directly, with no transform/middleware service.

• New format field on the webhook (json default · slack · discord), declared on the strict webhooks schema, validated on create/update, and returned in the (masked) hook responses.
• The dispatcher renders each task event into the target shape before signing/sending:
  • Slack → { text, blocks } (Block Kit: event + task key/name, status · priority · due context line)
  • Discord → { embeds: [...] } (title + status/priority/due fields)
  • json (and anything unknown) → the raw envelope unchanged
• The transformer (formatForTarget) is a pure function in webhookRules.js, covered by unit tests; the HMAC signature now covers exactly the bytes sent.

Usage: POST /api/v2/webhooks with { name, url: <slack/discord incoming-webhook URL>, events, format: "slack" }.

Verification

• Full jest suite passes (incl. new format-preset tests); webhooks module runtime-loads with STORAGE_TYPE set; transformer output spot-checked.

Note: this is backend-only (webhooks have no settings UI yet) — a webhook is configured via the API. A "Send to Slack/Discord" UI toggle can come with the webhooks settings page later.

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