Add Settings page with .env manager

- Sidebar: ⚙ Settings link pinned to bottom of nav
- Settings page: view all config keys (secrets masked), edit and save directly to .env
- Show/hide toggle for secret fields (tokens, keys)
- First-time setup banner with cp .env.example .env instructions when .env is missing
- Manual setup section with step-by-step terminal commands and where to find each credential
- New .env.example template with comments for all required variables
- Backend: GET/POST /api/settings/config router reads/writes mounted .env file
- docker-compose: mounts .env into backend container at /app/.env for write access

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

docker-compose.yml

@@ -22,6 +22,7 @@ services:
         condition: service_healthy
     volumes:
       - ./parsers:/app/parsers
+      - ./.env:/app/.env
 
   db:
     image: postgres:16-alpine