fix(api): chemins de bibliothèques — validation dédiée, alias et unicité

Validation des chemins externalisée (library-path) avec codes d'erreur
explicites, support d'alias LIBRARY_PATH_ALIASES pour traduire un chemin
hôte vers le montage conteneur, rejet des doublons de chemin entre
bibliothèques, documentation README/.env.example et docker-compose
paramétrable via READABOOK_LIBRARY_HOST_PATH.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yaml

@@ -11,11 +11,12 @@ services:
       STORAGE_DIR: /data/storage
       JWT_SECRET: ${JWT_SECRET:-dev-change-me-readabook}
       OPEN_LIBRARY_ENABLED: ${OPEN_LIBRARY_ENABLED:-true}
+      LIBRARY_PATH_ALIASES: ${READABOOK_LIBRARY_ALIAS_FROM:-/library}=/library
       INITIAL_ADMIN_EMAIL: ${INITIAL_ADMIN_EMAIL:-admin@readabook.local}
       INITIAL_ADMIN_PASSWORD: ${INITIAL_ADMIN_PASSWORD:-readabook-admin-change-me}
     volumes:
       - ./data:/data
-      - ./data/library:/library:ro
+      - /home/anthony/Documents/Projects/ReadaBook/Books:/library:ro
     healthcheck:
       test: ["CMD", "node", "-e", "fetch('http://127.0.0.1:3000/healthz').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"]
       interval: 10s