Add /api/v1/ip/<ip> endpoint for IP to AS lookups

- Add handleIPLookup handler that uses GetASInfoForIP
- Create writeJSONError and writeJSONSuccess helper functions
- Refactor all JSON error responses to use the helpers
- Add GetASInfoForIP to Store interface
- Add mock implementation for tests
- Fix all linter warnings

internal/database/interface.go

@@ -44,6 +44,9 @@ type Store interface {
 	GetPrefixDistribution() (ipv4 []PrefixDistribution, ipv6 []PrefixDistribution, err error)
 	GetLiveRouteCounts() (ipv4Count, ipv6Count int, err error)
 
+	// IP lookup operations
+	GetASInfoForIP(ip string) (*ASInfo, error)
+
 	// Lifecycle
 	Close() error
 }