feat: add per-user Pushover notifications

[steipete]

Replacement/fixed landing PR for #6. Shadow's original implementation is preserved as the base commit; this branch adds the maintainer fixups needed to land it cleanly on current main.

Fixups added here:

• rebase on current main after Fix chat realtime status and mobile nav #7 landed
• renumber SQLite migration to 0008_user_notification_settings.sql to avoid the existing 0006/0007 migrations
• add changelog entry with contributor credit
• add direct-DM recipient and notifier edge coverage to keep the 90% coverage gate green
• regenerate embedded web assets

Verification:

• go test ./... && pnpm coverage
• pnpm fmt:ts:check && pnpm lint && pnpm typecheck && pnpm -r typecheck && pnpm build && git diff --exit-code -- apps/api/internal/webassets/dist
• pnpm test:e2e

Supersedes #6.