Complete API-only mode implementation

[hellosunghyun]

Summary

This PR completes the API-only mode implementation for Invidious, allowing it to be built and run without GUI/frontend components. This significantly reduces the binary size and dependencies, making it ideal for API-focused deployments.

Motivation

The API-only mode was partially implemented but had several missing pieces that prevented it from working correctly. This PR fills those gaps and ensures the API-only build works seamlessly.

Changes

• Added conditional compilation flags ({% if flag?(:api_only) %}) throughout the codebase to exclude frontend-specific code
• Created stub implementations for database operations in API-only mode (DummyDB, VideoNotification, etc.)
• Updated Docker configurations to support API-only builds with proper build arguments
• Refactored require statements into a dedicated requires.cr file for better modularity
• Fixed route handlers to work correctly without frontend dependencies
• Added proper error handling for API-only mode

How to use

Using Crystal directly:

crystal build src/invidious.cr -Dapi_only --release

Using Make:

make API_ONLY=1

Using Docker:

# For AMD64
docker build --build-arg release=1 --build-arg api_only=1 -t invidious-api-only .

# For ARM64
docker build --platform linux/arm64 --build-arg release=1 --build-arg api_only=1 -f docker/Dockerfile.arm64 -t invidious-api-only:arm64 .

Benefits

• Reduced binary size: The API-only binary is significantly smaller without frontend assets and dependencies
• Lower resource usage: Less memory and CPU usage due to excluded frontend components
• Faster startup: Quicker initialization without frontend-related setup
• Microservice friendly: Perfect for API-only deployments in microservice architectures
• Container optimization: Smaller Docker images for cloud deployments

Testing

• Built and tested API-only mode on both AMD64 and ARM64 architectures
• Verified all API endpoints work correctly without frontend dependencies
• Confirmed database stubs work as expected in API-only mode
• Tested Docker builds with the new build arguments

Breaking changes

None. This change is fully backward compatible. The default build still includes all frontend components.

[SamantazFox]

What you propose here is much more than an "API only" mode. It's a whole "no database" and "no users" mode. Be aware that the API-only mode is meant to be a clipious/freetube backend (primarily a cloud account server) rather than a pure content proxy (for scrapping, yt-dlp might be more adapted).