Centralize visible issue filtering and Done retention

[joryirving]

Goal

Centralize visible issue filtering so Board and Projects use the same Done retention and closed-issue behavior.

Problem

Board and Projects currently have different issue visibility behavior. Board defaults to open issues only, while Projects can show completed/Done issues indefinitely. This makes Done inconsistent across views.

Desired behavior

• Open issues are visible by default.
• Recently closed/Done issues are visible for a retention window.
• Default Done retention is 7 days.
• Old closed/Done issues are hidden by default.
• `includeClosed=true` or equivalent can still show more history where supported.

Suggested implementation

• Add a shared helper/query builder for visible issue filtering.
• Example: `buildVisibleIssueWhere({ includeClosed, doneRetentionDays, filters })`.
• Use the helper in Board and Projects.
• Add optional `DISPATCH_DONE_RETENTION_DAYS`, default `7`, if low-risk.

Tests

Add/update tests for:

• open issues are visible
• recently closed issues appear in Done
• old closed issues are hidden by default
• includeClosed behavior still works if present
• Board and Projects use consistent filtering

Acceptance criteria

• Board and Projects share Done retention behavior.
• Done does not appear empty in one view and immortal in another.
• Default retention is 7 days.
• 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.