feat: Add multi-site product search system with SearXNG integration

This commit adds a comprehensive product search feature that allows users
to search for products across multiple configured e-commerce sites.

Backend changes:
- Add SearXNG service to docker-compose for meta-search capability
- Add SearchSite model and Alembic migration with default sites
- Create searxng_service.py for search queries across sites
- Create light_scraper_service.py for fast HTTP-based extraction
- Create search_service.py orchestrator combining SearXNG + HTTP + Browserless
- Add API endpoints for search (/api/search) with SSE streaming
- Add CRUD endpoints for search sites (/api/search-sites)

Frontend changes:
- Add new /search page with real-time results display
- Create SearchResultCard component for product display
- Add Progress, Checkbox, and Badge UI components
- Update Settings page with search sites management section
- Add navigation link to search page
- Create new modern SVG logo with price-flow gradient design
- Update favicon to SVG format

The search system uses a hybrid approach:
1. SearXNG finds product URLs across configured sites
2. Light HTTP scraper attempts fast extraction
3. Falls back to Browserless for JavaScript-heavy sites
4. Results stream progressively via SSE

docker-compose.yml

@@ -13,6 +13,7 @@ services:
     environment:
       - DATABASE_URL=postgresql://priceflow:priceflow@db:5432/priceflow
       - BROWSERLESS_URL=ws://browserless:3000
+      - SEARXNG_URL=http://searxng:8080
       - LOG_LEVEL=INFO
       - CORS_ORIGINS=*
     depends_on:
@@ -20,6 +21,8 @@ services:
         condition: service_healthy
       browserless:
         condition: service_started
+      searxng:
+        condition: service_started
     volumes:
       - screenshots_data:/app/screenshots
     networks:
@@ -47,6 +50,20 @@ services:
     networks:
       - nginx_default
 
+  # SearXNG - Moteur de recherche auto-hébergé
+  searxng:
+    image: searxng/searxng:latest
+    container_name: priceflow-searxng
+    restart: unless-stopped
+    ports:
+      - "8888:8080"
+    environment:
+      - SEARXNG_BASE_URL=http://searxng:8080
+    volumes:
+      - ./searxng:/etc/searxng:rw
+    networks:
+      - nginx_default
+
   # Browserless - Navigateur headless pour le scraping
   browserless:
     image: ghcr.io/browserless/chromium:latest