feat: Auto-queue new releases into download queue

[SloppyTurtle]

Overview

When the scheduler detects a new release (issue #3), optionally
auto-queue it for download using Shelfmark's existing download queue
infrastructure. Depends on #1 and #3.

Goal

Allow users to configure per-author or global auto-queue behavior so
that new releases are automatically sent to the download queue without
manual intervention.

Implementation

Auto-Queue Logic

• Runs after release detection in the scheduler job
• Checks watchlist_auto_queue setting (global) and per-author override
• If enabled, calls existing download queue logic for each pending release
• Updates release status: pending > queued
• On download completion: queued > downloaded
• On failure: stays queued, retries next interval up to configurable max retries

Per-Author Auto-Queue Override
Add auto_queue column to watchlist_authors:

• auto_queue TEXT - inherit, enabled, disabled. Default inherit (uses global setting)

Queue Integration

• Hook into existing download queue, do not rewrite it
• Use same source priority logic as manual queue
• Respect existing format preferences
• If no source found, leave as pending and retry next cycle

Failure Handling

• No source found: stay pending, retry next cycle
• Download fails: stay queued, increment retry count
• Max retries reached: set status to failed, notify user
• Source unreachable: skip, retry next cycle

New Settings

• watchlist_auto_queue (default false) - Global auto-queue toggle
• watchlist_max_retries (default 3) - Max download attempts per release
• watchlist_preferred_format (default ebook) - Preferred format when both available

Acceptance Criteria

• Auto-queue respects global setting
• Auto-queue respects per-author override (inherit/enabled/disabled)
• Release status transitions correctly (pending > queued > downloaded)
• Failed downloads retry up to max retries
• Releases with no source found stay pending and are retried
• Existing download queue logic is not modified
• Unit tests for status transition logic
• Unit tests for per-author override behavior
• No duplicate queue entries for the same release

Out of Scope

• Notifications (issue feat: Add notifications for new release detection and downloads #5)
• UI changes (covered in feat: Add watchlist UI #2)

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