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/discovery_record.py

@@ -0,0 +1,16 @@
+from sqlalchemy import Column, String, DateTime, Text
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from datetime import datetime
+from app.database import Base
+import uuid
+
+
+class DiscoveryRecord(Base):
+    __tablename__ = "discovery_records"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    user_id = Column(UUID(as_uuid=True), nullable=False, index=True)
+    product = Column(String(500), nullable=False)
+    market = Column(String(200), default="")
+    companies = Column(JSONB, default=[])
+    created_at = Column(DateTime, default=datetime.utcnow)