feat(analytics): explicit Column.isDone flag

[Musiker15]

Summary

Replaces the analytics "last column by position = done" heuristic with an explicit, user-pinnable Column.isDone flag. The heuristic silently skewed cycle time / lead time / throughput / CFD on any board that didn't end in a Done/Closed column (e.g. a trailing "Blocked" parking lot).

Closes the Column.isDone item from the v0.2.0-beta Deferred list.

Why no ADR

Column.isDone is a plain server-visible boolean, semantically identical to the existing Column.position / Column.wipLimit fields (neither of which has an ADR). It introduces no new threat-model surface — the server already sees column ordering and WIP limits. The behavioural change (heuristic → flag) is documented inline + in the CHANGELOG.

Changes

• Schema: Column.isDone Boolean @default(false) + hand-written migration 20260528000000_column_is_done (ALTER TABLE … ADD COLUMN … DEFAULT false).
• Repository / API: Columns.create + Columns.update accept isDone; POST /api/boards/[id]/columns and PATCH /api/columns/[id] validate it; ColumnDto carries it.
• UI (kanban-view.tsx): a ✓ toggle in each column header (aria-pressed, descriptive title) plus a DONE badge when flagged. Optimistic flip in board-client.tsx with revert-on-failure. New boards seed the default "Done" column with the flag on.
• Analytics: extracted selectDoneColumnIds into src/lib/analytics/done-columns.ts (pure, dependency-free). Prefers flagged columns; falls back to the legacy last-column heuristic only when nothing is flagged — so existing boards keep working without a data backfill.

Backward compatibility

Existing boards have isDone = false everywhere after the migration → the fallback keeps their analytics identical to today. The moment a user flags a column, the explicit set takes over.

Tests

tests/unit/done-columns.test.ts — 5 cases (empty, flagged set, flag overrides positional fallback, last-column fallback, fallback respects position not array order). pnpm typecheck + pnpm lint clean.

Test plan

• CI green (incl. the migration applying cleanly on a fresh DB)
• Manual: toggle a non-last column as Done, open Analytics, confirm Throughput / Cycle Time count cards entering that column
• Manual: a fresh board's "Done" column shows the DONE badge by default
• Manual: an old board with no flag still computes analytics off the last column

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