Fix assignment notification identities on access revoke

[Chris0Jeky]

Board-access revocation previously emitted card.assignments with an empty entity ID, including when no assignment changed. Preserve affected card IDs through the existing transaction and notify each card after commit.

Five SQLite tests verify multiple cards, archived assignments, retained owner eligibility, unchanged assignments, other boards and assignees, committed webhook metadata, and save/commit rollback. Full backend command dotnet test backend/Taskdeck.sln -c Release -m:1 --disable-build-servers: 9,564 passed, 34 existing skips, zero failures at 4c2f60c59. An independent review found no merge-blocking defect.

This reduces unnecessary consumer refreshes while preserving existing authorization and review-first behavior. Canonical delivery documentation is coordinated with the integration PR. Reliable delivery remains #3024. Live webhook HTTP delivery and PostgreSQL/Docker-dependent skipped cases were not exercised locally.

Closes #2979

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-12T15:28:43.139744Z	4c2f60c	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[Chris0Jeky]

Delivery is coordinated through integration PR #3050, which preserves this PR's exact source head and commits. It qualifies all reviewed product slices against main1931da65d after the Smart CI repair; no source merge-resolution changes were required. Local source evidence remains scoped to its recorded head, and the integration's hosted checks and bounded interaction review qualify the combined delivery. Existing review feedback is triaged once; this source PR will be reconciled after the integration lands.