fix(db): transactions wait for the write lock instead of failing (FLAW-23)

A load test with 16 concurrent writers lost 12 of 480 writes with
SQLITE_BUSY: transactions that read before writing cannot be helped by
busy_timeout once another connection writes. _txlock=immediate makes
every transaction take the write lock at BEGIN, so it queues instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/concurrency_test.go

@@ -0,0 +1,62 @@
+package storage
+
+import (
+	"fmt"
+	"path/filepath"
+	"sync"
+	"testing"
+
+	"github.com/sandrodach/mindsift/internal/domain"
+)
+
+// Many writers at once on one SQLite file (FLAW-23): transactions that read before they write must
+// wait for the write lock instead of failing with SQLITE_BUSY.
+func TestConcurrentWritersDoNotFail(t *testing.T) {
+	store, err := NewStore(filepath.Join(t.TempDir(), "load.db"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer store.Close()
+	u, _ := store.CreateUser("load@example.com", "hash", "Last", "member")
+	tl, _ := store.CreateTaskList(u.ID, "", "", "Last", "", "")
+	var ids []string
+	for i := 0; i < 20; i++ {
+		th, _ := store.CreateThoughtWithDetails(u.ID, fmt.Sprintf("Aufgabe %d", i), "normal", "manual", "", tl.ID, "todo", "")
+		_, _ = store.UpdateThoughtTriage(u.ID, th.ID, "keep", "task", "", "", tl.ID)
+		ids = append(ids, th.ID)
+	}
+
+	var wg sync.WaitGroup
+	errs := make(chan error, 400)
+	for w := 0; w < 16; w++ {
+		wg.Add(1)
+		go func(w int) {
+			defer wg.Done()
+			for i := 0; i < 15; i++ {
+				status := "todo"
+				if (w+i)%2 == 0 {
+					status = "done"
+				}
+				items := []domain.ThoughtReorderItem{{ID: ids[(w+i)%len(ids)], SortOrder: i, KanbanStatus: &status}}
+				if err := store.ReorderTasks(tl.ID, items); err != nil {
+					errs <- fmt.Errorf("reorder: %w", err)
+				}
+				if _, err := store.CreateThought(u.ID, fmt.Sprintf("Gedanke %d-%d", w, i), "normal", "manual", ""); err != nil {
+					errs <- fmt.Errorf("create: %w", err)
+				}
+			}
+		}(w)
+	}
+	wg.Wait()
+	close(errs)
+	failed := 0
+	for err := range errs {
+		if failed < 3 {
+			t.Error(err)
+		}
+		failed++
+	}
+	if failed > 0 {
+		t.Fatalf("%d of %d writes failed", failed, 16*15*2)
+	}
+}