Add discovery search history with auto-save, fix timeout causing search failure

- Save every search result to DB for later review
- Add '搜索历史' tab with timeline view, load/delete records
- Raise discovery search timeout from 30s to 120s (Bing Puppeteer needs ~40s)
- Reduce search queries from 4 to 3 for faster response
- New model: DiscoveryRecord (user_id, product, market, companies JSON)
- API: POST/GET/DELETE /api/v1/discovery/records
- Migration: discovery_records table

backend/app/models/__init__.py

@@ -16,6 +16,7 @@ from .certification import Certification, CertType, CertStatus
 from .invoice import Invoice, InvoiceType, InvoiceStatus
 from .referral import ReferralCode, Referral
 from .search_provider import SearchProvider
+from .discovery_record import DiscoveryRecord
 
 __all__ = [
     "User", "Product",
@@ -36,4 +37,5 @@ __all__ = [
     "Invoice", "InvoiceType", "InvoiceStatus",
     "ReferralCode", "Referral",
     "SearchProvider",
+    "DiscoveryRecord",
 ]