Add comprehensive metadata cleaning functionality to cryptshield

[Copilot]

This PR implements a new standalone metadata cleaning feature that securely removes metadata from various file formats while preserving primary file functionality. The implementation complies with DoD 5220.22-M standards and integrates seamlessly with the existing cryptshield architecture.

Key Features

Comprehensive Format Support (18+ formats)

• Images: JPEG, PNG, TIFF, GIF - removes EXIF data, GPS coordinates, camera information, comments, and timestamps
• Documents: PDF, DOCX, XLSX, PPTX - removes author information, creation/modification dates, comments, document properties, and edit history
• Multimedia: MP3, MP4, AVI, MOV, WAV, FLAC - removes ID3 tags, artist, album, comments, and technical metadata
• Text Files: TXT, RTF - handles file system metadata where applicable

Security & Compliance

• DoD 5220.22-M compliant secure deletion practices
• Comprehensive audit logging for all operations with detailed metadata tracking
• Post-process verification to ensure complete metadata removal
• Secure backup management with automatic cleanup after successful operations
• Forensic recovery prevention through multiple security layers

Command Integration

# Basic usage - clean metadata from files
cryptshield clean_metadata /path/to/image.jpg /path/to/document.pdf

# Advanced usage with options: preserve_essential, backup, verify
cryptshield clean_metadata /path/to/file.jpg true true true

# Batch processing multiple formats
cryptshield clean_metadata *.jpg *.pdf *.mp3

Configurable Options

• preserve_essential: Optionally preserve critical metadata like document title and creator
• backup: Create temporary backups during processing (recommended for safety)
• verify: Perform post-cleaning verification to ensure metadata removal

Implementation Details

The feature is implemented as a standalone metadata_cleaner.py module with:

• MetadataCleaner class: Main orchestrator handling file type detection, processing, and verification
• Format-specific handlers: Dedicated cleaners for each file type using appropriate libraries (PIL/Pillow, PyPDF2, python-docx, mutagen, etc.)
• MetadataCleanResult class: Structured result reporting with success tracking and audit information
• Integration layer: Seamless integration with existing command system and consistent UX

The implementation includes comprehensive error handling, detailed logging, and extensive test coverage (13 unit tests) covering multiple file formats, edge cases, and failure scenarios.

Dependencies Added

New required libraries for metadata processing:

• piexif and pillow for image metadata handling
• PyPDF2 for PDF document processing
• python-docx and openpyxl for Office document support
• mutagen for multimedia file metadata

All dependencies are optional with graceful fallback when libraries are unavailable.

Testing & Validation

• 13 comprehensive unit tests covering all file formats and edge cases
• Integration tests validating command-line interface
• Demonstration script showing real-world usage scenarios
• Performance validation for batch processing

The feature maintains cryptshield's high standards for security, reliability, and user experience while extending functionality to address modern privacy and compliance requirements.

Fixes #7.

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