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/models/server_log.py

@@ -0,0 +1,72 @@
+"""Server log model for audit trail."""
+from datetime import datetime
+from typing import Optional
+
+from app.extensions import db
+
+
+class ServerLog(db.Model):
+    """Server log model for tracking system events.
+    
+    Attributes:
+        id: Primary key
+        level: Log level (info, warning, error)
+        message: Log message content
+        timestamp: Event timestamp
+    """
+    __tablename__ = 'server_log'
+    
+    id = db.Column(db.Integer, primary_key=True)
+    level = db.Column(db.String(20), nullable=False, index=True, default='info')
+    message = db.Column(db.Text, nullable=False)
+    timestamp = db.Column(db.DateTime, default=datetime.utcnow, 
+                         nullable=False, index=True)
+    
+    def __repr__(self) -> str:
+        """String representation of ServerLog."""
+        return f'<ServerLog [{self.level.upper()}] {self.message[:50]}>'
+    
+    @classmethod
+    def log_info(cls, message: str) -> 'ServerLog':
+        """Create an info level log entry.
+        
+        Args:
+            message: Log message
+            
+        Returns:
+            ServerLog instance
+        """
+        log = cls(level='info', message=message)
+        db.session.add(log)
+        db.session.commit()
+        return log
+    
+    @classmethod
+    def log_warning(cls, message: str) -> 'ServerLog':
+        """Create a warning level log entry.
+        
+        Args:
+            message: Log message
+            
+        Returns:
+            ServerLog instance
+        """
+        log = cls(level='warning', message=message)
+        db.session.add(log)
+        db.session.commit()
+        return log
+    
+    @classmethod
+    def log_error(cls, message: str) -> 'ServerLog':
+        """Create an error level log entry.
+        
+        Args:
+            message: Log message
+            
+        Returns:
+            ServerLog instance
+        """
+        log = cls(level='error', message=message)
+        db.session.add(log)
+        db.session.commit()
+        return log