fix(tasks): preserve audit changes and paginate visible work

[mfethe1]

Summary

Task assignment, priority, and deadline changes currently lose their before/after values, and newer private tasks can hide accessible work from a limited task list. This change records each changed field in the same transaction as the task update, applies channel visibility before the database limit, and adds continuation cursors ordered by modification time and task ID. Retrying an unchanged patch leaves both history and the modification timestamp unchanged. Pagination remains live: a task modified between pages can move ahead of the cursor and requires a first-page refresh.

Migration 0047 adds structured task-event changes while preserving legacy rows. Deploy readers that understand the new action names before new writers; older relay binaries cannot read newly emitted action names.

Related issue

Builds on #10. Searched existing issues and PRs; no duplicate history/pagination fix found.

Testing

At head 8a35c1f14ae5f618ffbf416b1a6d1786e7b46f86, 126 database unit tests and a targeted 12-test isolated PostgreSQL CI lane passed (1,536 unrelated tests skipped), including desired-schema bootstrap and the 0046-to-0047 upgrade. A real signed HTTP flow exercised create, update, history, private-channel visibility, two-page retrieval, and retry behavior. Five targeted mutations were rejected.

Full just ci failed: 6,583 desktop JavaScript tests passed, then the mobile suite reported 2,241 passed and eight failed agent-activity golden comparisons. Those same eight comparisons failed at unchanged base adeec6eb5c4ff425aa03d8a744a295da4429f67f; all eight rendered actual PNGs were byte-identical between base and candidate. The rendering-platform explanation remains unconfirmed. This PR changes no mobile files or expected images. The broader just test launcher separately failed to start Docker Compose on Airy; the native database and HTTP lane ran independently.

Hosted runs for this head tested merge commit 39f3eb6651c91186f2d111d046add48a3cbb21a1. Image-cache writes failed, and the Windows Rust job reported 949 passed and 21 failed buzz-acp tests. Completed checks also report PostgreSQL, desktop smoke, and desktop integration failures. These gates remain failed on this PR; passing checks on separate repair PRs and green summary jobs do not qualify it for merge.

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