Remove the per-peer route tables (closes #122)

The routing table in memory now holds every route, so the two per-peer route
tables go, with every write, delete, count and read that served them. The
prefix handler applies each update to the table, writes the table's changes
every second, and stores each new prefix with its first-seen time. The peer
delete, its retry on the next flush and the updates held for it go too.

The image's memory settings fit a full table of about 1 GB: GOMEMLIMIT of
2.5 GiB, a 512 MiB page cache for the writing connection, and SQLite heap
limits of 768 MiB and 1 GiB. The README describes the new table, its
database copy and the memory budget.

Model: opus-5-5

internal/database/utils.go

@@ -1,8 +1,6 @@
 package database
 
 import (
-	"fmt"
-	"net"
 	"strings"
 
 	"github.com/google/uuid"
@@ -20,15 +18,3 @@ 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
-}