Add per-user Pushover notifications

[thewilloftheshadow]

Here's a good and quick way to get mobile notifications for this right now until such a time as mobile exists! (Also Pushover is fantastic for Codex notifications too :P)

Summary

• add per-user notification settings backed by a new user_notification_settings table
• add Pushover as the first push notifier, configured with CLICKCLACK_PUSHOVER_API_TOKEN
• expose Pushover opt-in/user-key fields through /api/me and the account settings UI
• notify opted-in non-authors for channel messages, thread replies, webhook/slash posts, and DMs

Verification

• go test ./apps/api/internal/config ./apps/api/internal/store/sqlite ./apps/api/internal/httpapi ./apps/api/cmd/clickclack
• pnpm --filter @clickclack/web typecheck
• pnpm --filter @clickclack/sdk-ts typecheck
• pnpm exec oxfmt --check --threads=1 apps/web/src/ChatApp.svelte apps/web/src/components/profile/ProfileSettingsModal.svelte apps/web/src/lib/types.ts packages/sdk-ts/src/generated/openapi.d.ts
• pnpm docs:site
• git diff --check

[steipete]

Thanks @thewilloftheshadow, this makes sense and I landed it via #8 because this PR branch was not maintainer-editable.

I kept your implementation as the base and added the maintainer fixups needed for current main: rebased after #7, renumbered the SQLite migration to 0008, restored the 90% coverage gate with DM-recipient/notifier tests, added the changelog entry, and regenerated embedded web assets. #8 is merged now with the GitHub Go, TypeScript, Playwright E2E, and Docker checks green.

Closing this PR as superseded by #8.