Add models and utils with type hints and optimizations

Models (6 + 1 association table):
- User: Authentication with bcrypt, admin role check, last_login tracking
- Player: Digital signage devices with auth codes, status tracking, online detection
- Group: Player/content organization with statistics properties
- Content: Media files with type detection, file size helpers, position ordering
- ServerLog: Audit trail with class methods for logging levels
- PlayerFeedback: Player status updates with error tracking
- group_content: Many-to-many association table for groups and content

Model improvements:
- Added type hints to all methods and properties
- Added database indexes on frequently queried columns (username, auth_code, group_id, player_id, position, level, timestamp, status)
- Added comprehensive docstrings
- Added helper properties (is_online, is_admin, file_size_mb, etc.)
- Added relationship back_populates for bidirectional navigation
- Added timestamps (created_at, updated_at, last_seen, uploaded_at)

Utils (4 modules):
- logger.py: Logging utility with level-based functions (info, warning, error)
- uploads.py: File upload handling with progress tracking, video optimization
- group_player_management.py: Player/group status tracking and bulk operations
- pptx_converter.py: PowerPoint to PDF conversion using LibreOffice

Utils improvements:
- Full type hints on all functions
- Comprehensive error handling
- Progress tracking for long-running operations
- Video optimization (H.264, 30fps, max 1080p, 8Mbps)
- Helper functions for time formatting and statistics
- Proper logging of all operations

Performance optimizations:
- Database indexes on all foreign keys and frequently filtered columns
- Lazy loading for relationships where appropriate
- Efficient queries with proper ordering
- Helper properties to avoid repeated calculations

Ready for template migration and testing

app/utils/__init__.py

@@ -0,0 +1,49 @@
+"""Utils package for digiserver-v2."""
+from app.utils.logger import log_action, log_info, log_warning, log_error, get_recent_logs
+from app.utils.uploads import (
+    save_uploaded_file,
+    process_video_file,
+    process_pdf_file,
+    get_upload_progress,
+    set_upload_progress,
+    clear_upload_progress,
+    get_file_size,
+    delete_file
+)
+from app.utils.group_player_management import (
+    get_player_status_info,
+    get_group_statistics,
+    assign_player_to_group,
+    bulk_assign_players_to_group,
+    get_online_players_count,
+    get_players_by_status
+)
+from app.utils.pptx_converter import pptx_to_pdf_libreoffice, validate_pptx_file
+
+__all__ = [
+    # Logger
+    'log_action',
+    'log_info',
+    'log_warning',
+    'log_error',
+    'get_recent_logs',
+    # Uploads
+    'save_uploaded_file',
+    'process_video_file',
+    'process_pdf_file',
+    'get_upload_progress',
+    'set_upload_progress',
+    'clear_upload_progress',
+    'get_file_size',
+    'delete_file',
+    # Group/Player Management
+    'get_player_status_info',
+    'get_group_statistics',
+    'assign_player_to_group',
+    'bulk_assign_players_to_group',
+    'get_online_players_count',
+    'get_players_by_status',
+    # PPTX Converter
+    'pptx_to_pdf_libreoffice',
+    'validate_pptx_file',
+]