This introduces a reusable system for serving images publicly without authentication, useful for external notification services like Gotify, Discord, or Telegram.

Features:
- Token-based public image access via /api/v1/publicimage endpoint
- No authentication required for accessing images with valid tokens
- Configurable expiration time (default: 24 hours, max: 31 days)
- Persistent storage in /config/public_images/ (survives restarts)
- Automatic cleanup of expired images (periodic + on startup)
- Validates file existence even if token not in memory (handles restarts)

Configuration options:
- public_base_url: External URL for Viseron (e.g., https://viseron.example.com)
- public_url_expiry_hours: How long public URLs remain valid (default: 24)

Technical details:
- Images stored with UUID token as filename
- Tokens stored in memory with expiration timestamp
- After restart, API falls back to disk lookup if token not in memory
- Hourly cleanup task removes expired tokens and files
- Startup cleanup removes orphaned files older than max expiry
- Supports JPG and PNG formats

Example usage:
```yaml
webserver:
  public_base_url: "https://viseron.example.com"
  public_url_expiry_hours: 48
```

- Remove unused datetime import in publicimage.py
- Fix type annotation for _cleanup_task (asyncio.Task | None)
- Simplify cleanup task cancellation to avoid unreachable code

Add configurable download limit for public image tokens to automatically
delete images after a specified number of downloads.

Changes:
- Add remaining_downloads field to PublicImageToken dataclass
- Implement download counter decrement in publicimage API
- Add automatic cleanup after final download
- Add public_url_max_downloads configuration parameter (default: 0 = unlimited)
- Update webserver documentation with usage examples

Configuration:
- 0 (default): Unlimited downloads
- 1: Single-use URL (perfect for notifications)
- N > 1: Limited sharing (delete after N downloads)