Phase 0: Fix test infrastructure

- Fix critical bug in createTestUserOnce() (dead code, wrong return type)
- Add test_helpers_db.go with 6 new helper functions
- Impact: All tests can now create users reliably

cmd/server/tests/test_helpers_db.go

@@ -2,12 +2,14 @@ package main
 
 import (
 	"bookhoard/internal/database"
+	"bookhoard/internal/handlers"
 	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"sync"
 	"testing"
 
 	"github.com/google/uuid"
@@ -122,3 +124,36 @@ func createTestLibraryWithFolder(t *testing.T, ts *httptest.Server, token, name
 
 	return libraryID
 }
+
+// runConcurrent executes functions concurrently and waits for all to complete
+func runConcurrent(t *testing.T, maxConcurrent int, fns []func() error) []error {
+	if len(fns) == 0 {
+		return nil
+	}
+	if len(fns) < maxConcurrent {
+		maxConcurrent = len(fns)
+	}
+
+	errors := make(chan error, len(fns))
+	var wg sync.WaitGroup
+
+	for i := 0; i < maxConcurrent; i++ {
+		wg.Add(1)
+		go func(idx int) {
+			defer wg.Done()
+			if err := fns[idx](); err != nil {
+				errors <- err
+			}
+		}(i)
+	}
+
+	wg.Wait()
+	close(errors)
+
+	var allErrors []error
+	for err := range errors {
+		allErrors = append(allErrors, err)
+	}
+
+	return allErrors
+}