Optimize database write performance

- Use SQLite UPSERT for UpdateLiveRoute to eliminate SELECT+UPDATE/INSERT pattern
- Add connection string optimizations (synchronous=NORMAL, cache_size)
- Add WAL checkpoint configuration for better write performance
- Add index on live_routes(id) for UPDATE operations
- Set WAL autocheckpoint to 1000 pages

These changes should reduce write amplification and improve overall
throughput by:
1. Reducing from 2 queries to 1 for route updates
2. Better WAL checkpoint management
3. More efficient UPDATE operations with dedicated index

internal/database/schema.sql

@@ -99,6 +99,10 @@ CREATE INDEX IF NOT EXISTS idx_live_routes_covering
 	ON live_routes(prefix_id, origin_asn_id, peer_asn, id)
 	WHERE withdrawn_at IS NULL;
 
+-- Index for UPDATE by id operations
+CREATE INDEX IF NOT EXISTS idx_live_routes_id
+	ON live_routes(id);
+
 -- Index for stats queries
 CREATE INDEX IF NOT EXISTS idx_live_routes_stats
 	ON live_routes(withdrawn_at)