perf(dashboard): index tasks.column/updatedAt and debounce detail fetches

Speeds up task-list load and interaction for projects with 100+ tasks.

- Migration 59 adds idxTasksColumn and idxTasksUpdatedAt; listTasks()
  filters by "column" on every board load and SSE/refresh paths sort by
  updatedAt, so each query was previously a full table scan plus a temp
  B-tree sort.
- Debounce handleEmbeddedOpenDetail in ListView so rapid keyboard/mouse
  navigation no longer fires a heavy /tasks/:id (log + comments) per
  selection; stale-target requests short-circuit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/dashboard-task-list-perf-100plus.md

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+---
+"@runfusion/fusion": patch
+---
+
+Speed up dashboard load and interaction for projects with 100+ tasks.
+
+Two cheap fixes that together cover the dominant hot paths:
+
+- **DB indexes on `tasks.column` and `tasks.updatedAt`** (migration 59 in `packages/core/src/db.ts`). `listTasks()` filters by `"column"` on every board load, and the SSE/refresh paths sort by `updatedAt`; neither column had an index, so each query did a full table scan plus a temp B-tree sort. With 100+ tasks this becomes the dominant cost on initial load.
+- **Debounce embedded detail-pane fetches** (`packages/dashboard/app/components/ListView.tsx`). `handleEmbeddedOpenDetail` previously fired a full `fetchTaskDetail` (which pulls log + comments) synchronously on every selection change, so rapid keyboard/mouse navigation through a long list would issue a burst of heavy requests. Fetches are now debounced to 200 ms and stale-target requests short-circuit before hitting the server and before applying state.