feat(backend): add PoolProxy, Querier, and per-test transaction infrastructure

New types and helpers for context-aware DB routing and per-test transactions:

- PoolProxy: wraps pgxpool.Pool, routes queries through context transaction when active
- Querier: interface accepted by fixture/helper functions for decoupling
- ContextWithTx / TxFromContext: store/extract pgx.Tx in context.Context
- SetupTestTx (in testutils): begins tx, stores in context, auto-rolls back on cleanup
- SetupTestTx (in testtx): package-level variant with semaphore for parallel safety

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/testutils/testtx/tx.go

@@ -0,0 +1,32 @@
+//go:build test
+// +build test
+
+package testtx
+
+import (
+	"context"
+	"testing"
+
+	"crussell/db"
+)
+
+// txSemaphore limits concurrent test transactions per package.
+var txSemaphore = make(chan struct{}, 8)
+
+// SetupTestTx begins a transaction, stores it in context, and auto-rolls back.
+func SetupTestTx(t *testing.T) (context.Context, db.Querier) {
+	t.Helper()
+	txSemaphore <- struct{}{}
+	pool := db.Conn.Pool()
+	tx, err := pool.Begin(context.Background())
+	if err != nil {
+		<-txSemaphore
+		t.Fatalf("SetupTestTx: failed to begin transaction: %v", err)
+	}
+	ctx := db.ContextWithTx(context.Background(), tx)
+	t.Cleanup(func() {
+		tx.Rollback(context.Background())
+		<-txSemaphore
+	})
+	return ctx, tx
+}