Serve the /api/v1/stats prefix distribution from memory (closes #30)

The prefix distribution was the last query on the stats path. It read one
index entry per live route on every request, so on a large database it took
the whole 4-second deadline and /api/v1/stats answered 500.

The distribution now lives next to the in-memory counts: seeded once at
startup from the same query, then kept exact by the live-route writes. A new
route whose prefix had no live route adds one at its mask length; a delete
that leaves a prefix with no live route takes one away. Each check is one
lookup on the prefix index, made only for new and removed routes, inside the
write.

Model: opus-5-5

internal/database/utils.go

@@ -1,6 +1,8 @@
 package database
 
 import (
+	"fmt"
+	"net"
 	"strings"
 
 	"github.com/google/uuid"
@@ -18,3 +20,14 @@ func detectIPVersion(prefix string) int {
 
 	return ipVersionV4
 }
+
+// prefixMaskLength returns the mask length of a prefix such as 192.0.2.0/24.
+func prefixMaskLength(prefix string) (int, error) {
+	_, network, err := net.ParseCIDR(prefix)
+	if err != nil {
+		return 0, fmt.Errorf("invalid prefix %s: %w", prefix, err)
+	}
+	maskLength, _ := network.Mask.Size()
+
+	return maskLength, nil
+}