feat(dataplane): record delivery attempt request/response timestamps

[mekilis]

Summary

• Record per delivery attempt requested_at (when the request is sent) and responded_at (when the response is received). responded_at is null when the endpoint returns no HTTP response (network error or timeout).
• Add acknowledged_at, requested_at and responded_at to eventdelivery.* meta event payloads so the pushed payload matches the delivery attempt polling API.
• Surface delivery_mode in the dashboard: a selector on the subscription form, and a delivery mode indicator on the event delivery details page.

Details

• New migration adds nullable requested_at/responded_at columns to delivery_attempts. The partition and unpartition rebuilds copy the new columns so existing data is preserved.
• The worker threads the dispatch start time and measured response time into the attempt on both the first delivery and retry paths.
• Legacy attempts created before the migration have null timestamps; the dashboard hides the fields when they are absent.

Test plan

• unit: parseAttemptFromResponse for success, response-with-error-status, and no-response (responded_at null)
• unit: meta event payload includes acknowledged_at, requested_at, responded_at
• integration: attempt persistence round trip and partition parity for the new columns
• manual: success (200), failure-with-response (500), no-response, 3xx and 4xx; verified timestamps on the polling API and in the live meta event body
• dashboard builds (AOT); delivery mode selector and details indicator render

---

Note

Medium Risk
Touches the core webhook delivery worker and delivery_attempts schema/partition migrations; nullable columns and tests limit blast radius, but incorrect timestamps could affect observability and meta-event consumers.

Overview
Adds requested_at and responded_at on delivery attempts end-to-end: nullable DB columns (migration + partition rebuilds), worker capture on initial delivery and retry (responded_at only when an HTTP status is received), persistence/API exposure, and OpenAPI updates.

Meta events now include acknowledged_at on the delivery envelope and the attempt’s requested_at / responded_at in eventdelivery.* payloads so webhook consumers match the polling API.

The dashboard gains delivery mode configuration on subscriptions and an at most once indicator on event delivery details, plus Requested/Responded timestamps on the attempt panel when present (legacy rows stay hidden).

^{Reviewed by Cursor Bugbot for commit 3321bcb. Bugbot is set up for automated code reviews on this repo. Configure here.}