feat: Add notifications for new release detection and downloads

[SloppyTurtle]

Overview

Notify users when new releases are detected for watched authors and
when auto-queued downloads complete or fail. Uses existing Shelfmark
notification infrastructure.

Goal

Leverage the existing notification system to alert users about watchlist
activity without building new notification infrastructure.

Notification Events

• watchlist.release_detected - New release found for watched author (default enabled)
• watchlist.download_complete - Auto-queued release downloaded successfully (default enabled)
• watchlist.download_failed - Release failed max retries (default enabled)
• watchlist.check_failed - Provider API unreachable during scheduled check (default disabled)

Notification Content

Release Detected
New release detected: {title} by {author}
Format: {format} | Expected: {release_date or Unknown}

Download Complete
Download complete: {title} by {author}
Added to your library.

Download Failed
Download failed: {title} by {author}
{retry_count} attempts failed. Manual intervention required.

New Settings

• watchlist_notify_detected (default true)
• watchlist_notify_complete (default true)
• watchlist_notify_failed (default true)
• watchlist_notify_check_failed (default false)

Acceptance Criteria

• Notifications fire for all four event types
• Each notification type can be toggled independently
• Notifications respect existing per-user notification preferences
• Notification content includes relevant book/author info
• No new notification infrastructure added, uses existing system only
• Unit tests for notification trigger logic

Depends On

• feat: Add watchlist database schema and backend API #1 watchlist database schema and backend API
• feat: Add watchlist UI #3 APScheduler and new release detection job
• feat: Auto-queue new releases into download queue #4 Auto-queue new releases into download queue