refactor: Unify leggen and leggend packages into single leggen package

- Merge leggend API components into leggen (api/, services/, background/)
- Replace leggend command with 'leggen server' subcommand
- Consolidate configuration systems into leggen.utils.config
- Update environment variables: LEGGEND_API_URL -> LEGGEN_API_URL
- Rename LeggendAPIClient -> LeggenAPIClient
- Update all documentation, Docker configs, and compose files
- Fix all import statements and test references
- Remove duplicate utility files and clean up package structure

All tests passing (101/101), linting clean, server functionality preserved.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/conftest.py

@@ -7,8 +7,8 @@ from pathlib import Path
 from unittest.mock import patch
 from fastapi.testclient import TestClient
 
-from leggend.main import create_app
-from leggend.config import Config
+from leggen.commands.server import create_app
+from leggen.utils.config import Config
 
 
 @pytest.fixture