Add periodic WAL checkpointing to fix slow queries

The WAL file was growing to 700MB+ which caused COUNT(*) queries to
timeout. Reads must scan the WAL to find current page versions, and
a large WAL makes this slow.

Add Checkpoint method to database interface and run PASSIVE checkpoints
every 30 seconds via the DBMaintainer. This keeps the WAL small and
maintains fast read performance under heavy write load.

internal/database/interface.go

@@ -87,6 +87,7 @@ type Store interface {
 	// Maintenance operations
 	Vacuum(ctx context.Context) error
 	Analyze(ctx context.Context) error
+	Checkpoint(ctx context.Context) error
 }
 
 // Ensure Database implements Store