Unify IP lookup response structure and add PTR lookups

- Always return consistent JSON structure with query and results array
- Add PTR field to IPInfo for reverse DNS records
- Support comma-separated IPs and hostnames in single query
- Do PTR lookup for all IPs (direct, resolved from hostname, or listed)
- Remove trailing dots from PTR records

internal/database/models.go

@@ -100,11 +100,12 @@ type ASInfo struct {
 
 // IPInfo represents comprehensive IP information for the /ip endpoint
 type IPInfo struct {
-	IP         string `json:"ip"`
-	Netblock   string `json:"netblock"`
-	MaskLength int    `json:"mask_length"`
-	IPVersion  int    `json:"ip_version"`
-	NumPeers   int    `json:"num_peers"`
+	IP         string   `json:"ip"`
+	PTR        []string `json:"ptr,omitempty"`
+	Netblock   string   `json:"netblock"`
+	MaskLength int      `json:"mask_length"`
+	IPVersion  int      `json:"ip_version"`
+	NumPeers   int      `json:"num_peers"`
 	// AS information
 	ASN         int    `json:"asn"`
 	ASName      string `json:"as_name,omitempty"`