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

Looking up an address on /ip/ and /api/v1/ip/ read every IPv6 live route and about half of the IPv4 range index, so on a day-sized database it passed the 30-second request timeout. The lookup is now one function for both families: from the longest mask length down, it looks up the address's network prefix on the existing prefix index, and the first live route wins.

The feed sends IPv6 withdrawals uncompressed, so they never matched the stored compressed prefix and never removed a route. Prefixes are now stored in the text form net/netip prints, so IPv6 withdrawals take effect.

ip_start, ip_end and GetASInfoForIP are removed; nothing read them. A database from before this change must be deleted.

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"`