Record origin changes and show them on the prefix page (closes #161)

The routing table records each prefix and origin AS pair's origin changes:
when it gains its first peer and when it loses its last. Each row holds the
prefix, the origin AS, announced or withdrawn, the peer count and the time.
The prefix handler writes them to a new origin_changes table, and the database
maintainer prunes each prefix to its newest 500 rows and 72 hours. The prefix
page lists them newest first, at most 5,000.

Model: deepseek-v4-pro

internal/database/interface.go

@@ -55,6 +55,14 @@ type Store interface {
 	WriteRouteChanges(changes []routetable.Change, newPeers map[string]uint) error
 	ReadRouteTable(ctx context.Context) (*routetable.Table, error)
 
+	// Origin change operations: the prefix and origin AS pairs' first and
+	// last peers, as the routing table records them.
+	WriteOriginChanges(changes []routetable.OriginChange) error
+	GetOriginChanges(
+		ctx context.Context, prefix netip.Prefix, limit int,
+	) ([]routetable.OriginChange, error)
+	PruneOriginChanges(ctx context.Context) error
+
 	// ASN WHOIS operations
 	GetNextStaleASN(ctx context.Context, staleThreshold time.Duration) (int, error)
 	UpdateASNWHOIS(ctx context.Context, update *ASNWHOISUpdate) error