Complete rewrite of app.py with OOP architecture and GitHub secrets integration

[Copilot]

The existing youtube_stats.py had critical bugs (undefined hours_watched variable causing crashes), no error handling, and poor code structure. This PR replaces it with a production-ready app.py that properly uses YOUTUBE_API_KEY from GitHub secrets.

Architecture

6 classes with single responsibilities:

• Config - Centralized configuration with validation
• YouTubeAPIClient - API calls with retry logic (3 attempts, exponential backoff)
• DataManager - CSV storage, automated backups (30-day retention), integrity checks
• ChartGenerator - Matplotlib visualizations with proper backend configuration
• ReadmeUpdater - Documentation sync
• YouTubeStatsApp - Main orchestrator

Key Improvements

API Integration:

• Reads YOUTUBE_API_KEY from environment (GitHub secrets in workflow)
• Retry logic with 10s timeout per request
• Handles pagination for 5000+ playlists (safety limit prevents infinite loops)
• Enhanced error messages with HTTP codes and URLs

Reliability:

• Data integrity checks (alerts on >50% changes)
• Automatic backups before writes
• Graceful degradation on non-critical failures

Code Quality:

• Full type hints (PEP 484)
• Comprehensive docstrings
• 11 unit tests (100% pass rate)
• CodeQL scan: 0 vulnerabilities

Example Usage

# Old: Procedural script with hardcoded values
API_KEY = os.getenv("YOUTUBE_API_KEY")
response = requests.get(url)  # No retry, no timeout

# New: Modular with proper error handling
client = YouTubeAPIClient(api_key, timeout=10, max_retries=3)
stats = client.get_channel_statistics(channel_id)  # Retries, validates, logs

Migration

Workflow updated to use app.py instead of youtube_stats.py. Both files can coexist; youtube_stats.py remains for backward compatibility but is superseded by this implementation.

Original prompt

revise and improve drastically the app.py then edit and fix by using the YOUTUBE_API_KEY from GitHub secrets.

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