feat: define paused member card behavior

[bionicman]

Description

Paused workspace members currently remain available for new card assignments
and mentions, with no indication that they are paused. Hiding them everywhere
would create the opposite problem: existing ownership and historical comments
would become difficult to understand.

This change treats pause as an eligibility state rather than a history rewrite:

• paused members cannot be newly assigned or mentioned;
• existing assignments survive and can still be removed;
• selected paused assignees remain visible with a Paused label and muted
  avatar;
• board filters retain paused members only while they are assigned somewhere on
  that board, place them after current members, and identify their status;
• comment attribution remains readable and gains a compact status label;
• reactivation restores normal eligibility without changing historical data;
• invited keeps its existing behavior and is not treated as paused.

The API enforces assignment eligibility so non-UI clients cannot bypass it.
Member status is included in existing card and board payloads; the UI does not
add per-comment or per-avatar requests.

Type of change

• Bug fix
• Feature (requires an approved issue — see below)
• Refactor / chore
• Documentation

Checklist

• I have linked the related issue below
• My code follows the existing style and conventions
• I have tested my changes locally
• I have included screenshots for any UI changes

Testing

• pnpm --filter @kan/api exec vitest run src/routers/card-members.test.ts
• pnpm --filter @kan/web exec vitest run src/views/board/memberFilters.test.ts
• pnpm --filter @kan/db typecheck
• pnpm --filter @kan/api exec tsc --noEmit --jsx preserve

Linked issue

Closes #575.

The workspace-scoping prerequisite was merged separately in #576.

The unit-test mock correction required by the description-normalization
baseline was merged in #585. This branch is now rebased onto that fixed
baseline.

Screenshots

The screenshots use a synthetic Middle-earth workspace. Boromir is a retained
historical assignee after being paused; Faramir is the active assignee taking
over the work.

Card member selector and historical comments

Board member filter