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>

frontend/Dockerfile

@@ -25,7 +25,7 @@ COPY --from=builder /app/dist /usr/share/nginx/html
 COPY default.conf.template /etc/nginx/templates/default.conf.template
 
 # Set default API backend URL (can be overridden at runtime)
-ENV API_BACKEND_URL=http://leggend:8000
+ENV API_BACKEND_URL=http://leggen-server:8000
 
 # Expose port 80
 EXPOSE 80

frontend/README.md

@@ -93,7 +93,7 @@ The frontend supports configurable API URLs through environment variables:
 
 - Uses relative URLs (`/api/v1`) that nginx proxies to the backend
 - Configure nginx proxy target via `API_BACKEND_URL` environment variable
-- Default: `http://leggend:8000`
+- Default: `http://leggen-server:8000`
 
 **Docker Compose:**