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/db/db_test.go

@@ -19,9 +19,9 @@ func resetEnv() {
 }
 
 func closePool() {
-	if DB != nil {
-		DB.Close()
-		DB = nil
+	if Conn != nil {
+		Conn.Pool().Close()
+		Conn = nil
 	}
 }
 
@@ -41,8 +41,8 @@ func TestConnect_Success(t *testing.T) {
 	}
 	defer closePool()
 
-	if DB == nil {
-		t.Fatal("DB is nil after successful Connect")
+	if Conn == nil {
+		t.Fatal("Conn is nil after successful Connect")
 	}
 }
 
@@ -56,14 +56,14 @@ func TestConnect_PingViaTestDB(t *testing.T) {
 	}
 	defer closePool()
 
-	conn, err := DB.Acquire(context.Background())
+	poolConn, err := Conn.Acquire(context.Background())
 	if err != nil {
 		t.Fatalf("Acquire failed: %v", err)
 	}
-	defer conn.Release()
+	defer poolConn.Release()
 
 	var result int
-	err = conn.QueryRow(context.Background(), "SELECT 1").Scan(&result)
+	err = poolConn.QueryRow(context.Background(), "SELECT 1").Scan(&result)
 	if err != nil {
 		t.Fatalf("Ping query failed: %v", err)
 	}
@@ -127,15 +127,15 @@ func TestConcurrentQueries(t *testing.T) {
 		wg.Add(1)
 		go func(id int) {
 			defer wg.Done()
-			conn, err := DB.Acquire(context.Background())
+			poolConn, err := Conn.Acquire(context.Background())
 			if err != nil {
 				errs <- fmt.Errorf("goroutine %d: acquire: %w", id, err)
 				return
 			}
-			defer conn.Release()
+			defer poolConn.Release()
 
 			var result int
-			err = conn.QueryRow(context.Background(), "SELECT $1::int", id).Scan(&result)
+			err = poolConn.QueryRow(context.Background(), "SELECT $1::int", id).Scan(&result)
 			if err != nil {
 				errs <- fmt.Errorf("goroutine %d: query: %w", id, err)
 				return