Add headless CLI and API

[jttdev]

Summary

• add a standalone Java 17 CLI and local HTTP API for catalog search, exact entry lookup, downloads, history, and subscriptions
• provide atomic progressive and HLS downloads with bounded network/process timeouts and source-aware library deduplication
• include OpenAPI documentation, example subscription config, systemd units, and regression coverage

Testing

• JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 ./mvnw -f headless/pom.xml clean verify
• 17 tests passed