Fix Firebase initialization crash and add .env.example configuration

[aacritea]

Summary

This PR fixes an issue where the application crashed during startup when FIREBASE_STORAGE_BUCKET was not configured. It also improves developer onboarding by adding a clearer .env.example configuration.

Problem

The Firebase service was initialized at import time in video_routes.py. If FIREBASE_STORAGE_BUCKET was not set, the application crashed during startup, preventing local development and testing.

Solution

• Implemented conditional Firebase initialization
• Added warning log when Firebase configuration is missing
• Ensured application starts successfully without Firebase configuration
• Added/updated .env.example to guide developers on required environment variables

Changes

• Updated Firebase initialization logic in routes/video_routes.py
• Added clearer environment variable configuration in .env.example
• Updated .gitignore where necessary

Impact

• Prevents application startup crashes
• Improves developer experience
• Enables easier local development
• Improves application robustness

Testing

• Verified application starts successfully without FIREBASE_STORAGE_BUCKET
• Confirmed Firebase-dependent endpoint raises appropriate error when configuration is missing