Add IP address search feature with global search box

- Add API endpoint GET /api/scans/by-ip/{ip_address} to retrieve
  last 10 scans containing a specific IP
- Add ScanService.get_scans_by_ip() method with ScanIP join query
- Add search box to global navigation header
- Create dedicated search results page at /search/ip
- Update API documentation with new endpoint

app/web/services/scan_service.py

@@ -260,6 +260,29 @@ class ScanService:
 
         return status_info
 
+    def get_scans_by_ip(self, ip_address: str, limit: int = 10) -> List[Dict[str, Any]]:
+        """
+        Get the last N scans containing a specific IP address.
+
+        Args:
+            ip_address: IP address to search for
+            limit: Maximum number of scans to return (default: 10)
+
+        Returns:
+            List of scan summary dictionaries, most recent first
+        """
+        scans = (
+            self.db.query(Scan)
+            .join(ScanIP, Scan.id == ScanIP.scan_id)
+            .filter(ScanIP.ip_address == ip_address)
+            .filter(Scan.status == 'completed')
+            .order_by(Scan.timestamp.desc())
+            .limit(limit)
+            .all()
+        )
+
+        return [self._scan_to_summary_dict(scan) for scan in scans]
+
     def cleanup_orphaned_scans(self) -> int:
         """
         Clean up orphaned scans that are stuck in 'running' status.