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>

compose.dev.yml

@@ -8,13 +8,13 @@ services:
     ports:
       - "127.0.0.1:3000:80"
     environment:
-      - API_BACKEND_URL=${API_BACKEND_URL:-http://leggend:8000}
+      - API_BACKEND_URL=${API_BACKEND_URL:-http://leggen-server:8000}
     depends_on:
-      leggend:
+      leggen-server:
         condition: service_healthy
 
   # FastAPI backend service
-  leggend:
+  leggen-server:
     build:
       context: .
       dockerfile: Dockerfile