chore: remove config.yaml mount from docker-compose; update config example

- Replace individual volume mounts (playbook_cache, plugins) with single app_data:/data mount
- Change FABLEDNETMON_DATABASE__URL to FABLEDNETMON_DATABASE_URL (underscore to hyphen)
- Simplify config.example.yaml to focus on required database URL setting
- Document that other settings are now managed via the Settings UI

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -4,11 +4,9 @@ services:
     ports:
       - "5000:5000"
     volumes:
-      - ./config.yaml:/config/config.yaml:ro
-      - playbook_cache:/data/playbook_cache
-      - plugins:/data/plugins
+      - app_data:/data
     environment:
-      - FABLEDNETMON_DATABASE__URL=postgresql+asyncpg://fablednetmon:fablednetmon@db/fablednetmon
+      - FABLEDNETMON_DATABASE_URL=postgresql+asyncpg://fablednetmon:fablednetmon@db/fablednetmon
     depends_on:
       db:
         condition: service_healthy
@@ -31,5 +29,4 @@ services:
 
 volumes:
   pgdata:
-  playbook_cache:
-  plugins:
+  app_data: