feat(ledger): add append-only double-entry engine

The Lightning bridge is modelled as the boundary with the outside
world and is the one account permitted to go negative; its negative
balance is exactly what is owed to players inside the system. All
other accounts are floored at zero by both the application and a
database trigger.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

go.mod

@@ -1,3 +1,13 @@
 module github.com/drjones/quantum-arcade
 
 go 1.26.5
+
+require github.com/jackc/pgx/v5 v5.10.0
+
+require (
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+	golang.org/x/text v0.29.0 // indirect
+)