Simplify peerings table to store AS numbers directly

- Rename asn_peerings table to peerings
- Change columns from from_asn_id/to_asn_id to as_a/as_b (integers)
- Remove foreign key constraints to asns table
- Update RecordPeering to use AS numbers directly
- Add validation in RecordPeering to ensure:
  - Both AS numbers are > 0
  - AS numbers are different
  - as_a is always lower than as_b (normalized)
- Update PeeringHandler to no longer need ASN cache
- Simplify the code by removing unnecessary ASN lookups

internal/routewatch/app_integration_test.go

@@ -2,6 +2,7 @@ package routewatch
 
 import (
 	"context"
+	"fmt"
 	"strings"
 	"sync"
 	"testing"
@@ -119,11 +120,16 @@ func (m *mockStore) RecordAnnouncement(_ *database.Announcement) error {
 }
 
 // RecordPeering mock implementation
-func (m *mockStore) RecordPeering(fromASNID, toASNID string, _ time.Time) error {
+func (m *mockStore) RecordPeering(asA, asB int, _ time.Time) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
-	key := fromASNID + "_" + toASNID
+	// Normalize
+	if asA > asB {
+		asA, asB = asB, asA
+	}
+
+	key := fmt.Sprintf("%d_%d", asA, asB)
 	if !m.Peerings[key] {
 		m.Peerings[key] = true
 		m.PeeringCount++