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/database/interface.go

@@ -29,7 +29,7 @@ type Store interface {
 	RecordAnnouncement(announcement *Announcement) error
 
 	// Peering operations
-	RecordPeering(fromASNID, toASNID string, timestamp time.Time) error
+	RecordPeering(asA, asB int, timestamp time.Time) error
 
 	// Statistics
 	GetStats() (Stats, error)