Debounce GitHub sync after Board card moves

[joryirving]

Goal

Keep the nice UX where moving a card eventually syncs with GitHub, but avoid calling `POST /api/sync` immediately after every single card move.

Problem

Dragging a card currently calls the move endpoint, then triggers a full issue sync. This is useful from a user perspective because the board catches up without requiring a manual Sync Issues click, but it can be too heavy and now collides with protected sync auth.

Desired behavior

• Moving a card should update Dispatch immediately.
• The UI should refresh board state from Dispatch after the move.
• GitHub issue sync should still happen automatically, but debounced/rate-limited/batched.
• Multiple card moves in a short window should result in one sync, not one sync per move.
• Sync failures should be surfaced gently, not silently swallowed.

Suggested implementation

• Replace immediate `POST /api/sync` after every move with a debounced sync request.
• Consider a 5-15 second debounce window.
• Prefer syncing only the affected repo if possible.
• If user auth is not implemented yet, avoid calling protected sync directly from client-side JavaScript unless there is a safe server-side/operator-auth path.
• Refresh board state from `/api/issues` after successful moves.

Tests

Add/update tests for:

• card move updates Dispatch/board state
• multiple moves only schedule one sync within debounce window
• sync is repo-scoped if implemented
• sync failure displays a warning
• no agent token is exposed to client-side JavaScript

Acceptance criteria

• Moving a card still feels automatic to the user.
• User does not need to manually click Sync Issues after every move.
• Multiple moves are batched/debounced.
• Board state refreshes from Dispatch after move.
• GitHub sync behavior is auth-safe.
• Tests pass.

Validation

Run:

• `npm ci`
• `npx prisma generate`
• `npm run lint`
• `npm run typecheck`
• `npm run test`
• `npm run build`

Do not cut a release in this task.