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/tx.go

@@ -0,0 +1,52 @@
+//go:build test
+// +build test
+
+package testutils
+
+import (
+	"context"
+	"testing"
+
+	"crussell/db"
+)
+
+// txSemaphore limits concurrent test transactions per package to prevent
+// pgxpool connection exhaustion when many tests use t.Parallel().
+// The cap (8) should match the -parallel N flag used in test invocations
+// and should be well below MaxConns per pool (currently 16).
+var txSemaphore = make(chan struct{}, 8)
+
+// SetupTestTx begins a transaction on the db.Conn pool, stores it in the
+// returned context (for PoolProxy routing), and returns the raw tx for use
+// with fixture functions (e.g. fixtures.CreateTestUser(tx)).
+//
+// The transaction is automatically rolled back in t.Cleanup. After calling
+// SetupTestTx, the test should call t.Parallel() and pass the returned context
+// to any request helper functions (MakeRequest, makeAdminRequest, etc.).
+//
+// Example:
+//
+//	func TestMyHandler(t *testing.T) {
+//	    t.Parallel()
+//	    ctx, tx := testutils.SetupTestTx(t)
+//	    userID, _ := fixtures.CreateTestUser(tx)
+//	    w := testutils.MakeUserRequest(handler, "GET", "/api/foo", nil, userID, ctx)
+//	    assert.Equal(t, 200, w.Code)
+//	}
+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
+}
+