refactor(backend): update test files for PoolProxy and per-test transactions

Migrate all test files from SetupTestDB/db.DB pattern to per-test transactions:

- Replace SetupTestDB(t) with SetupTestTx(t) for context + transaction
- Replace db.DB.Query/QueryRow/Exec with tx.Query/QueryRow/Exec
- Replace context.Background() with context from SetupTestTx
- Replace defer rows.Close() pattern with explicit rows.Close()
- Add testdb.SeedBaseline(pool) to all TestMain functions
- Wire db.Conn = db.NewPoolProxy(pool) in all TestMain functions

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

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

backend/auth/testmain_test.go

@@ -15,7 +15,7 @@ func TestMain(m *testing.M) {
 	InitJWT("test-secret-key-for-jwt-test")
 
 	pool := testdb.CreateTestDatabase("crussell_test_auth")
-	db.DB = pool
+	db.Conn = db.NewPoolProxy(pool)
 
 	code := m.Run()
 	testdb.DestroyTestDatabase(pool, "crussell_test_auth")