Look up IP addresses by prefix at each mask length (closes #48)

Looking up an address read every IPv6 live route and, for IPv4, about
half of the range index. Both families now look the address's prefix up
at each mask length, longest first, on the prefix index: at most 33 or
129 indexed lookups, however many routes are live.

The feed sends IPv6 withdrawals uncompressed but announcements
compressed, so the prefix handler now stores every prefix in the text
form net/netip prints, the form the lookup builds. IPv6 withdrawals now
remove their routes.

The IPv4 range columns, their index and the code that fills them are
removed, and so are the unused GetASInfoForIP lookups.

Model: opus-5-5

internal/database/models.go

@@ -77,9 +77,6 @@ type LiveRoute struct {
 	ASPath      []int     `json:"as_path"`
 	NextHop     string    `json:"next_hop"`
 	LastUpdated time.Time `json:"last_updated"`
-	// IPv4 range fields for fast lookups (nil for IPv6)
-	V4IPStart *uint32 `json:"v4_ip_start,omitempty"`
-	V4IPEnd   *uint32 `json:"v4_ip_end,omitempty"`
 }
 
 // PrefixDistribution represents the distribution of prefixes by mask length
@@ -88,16 +85,6 @@ type PrefixDistribution struct {
 	Count      int `json:"count"`
 }
 
-// ASInfo represents AS information for an IP lookup (legacy format)
-type ASInfo struct {
-	ASN         int       `json:"asn"`
-	Handle      string    `json:"handle"`
-	Description string    `json:"description"`
-	Prefix      string    `json:"prefix"`
-	LastUpdated time.Time `json:"last_updated"`
-	Age         string    `json:"age"`
-}
-
 // IPInfo represents comprehensive IP information for the /ip endpoint
 type IPInfo struct {
 	IP         string   `json:"ip"`