feat: implement Havenllo single-board kanban (Go+SQLite backend, Preact/Vite frontend)

- Single fixed 'Haven' board: lists (To do/In progress/Done) + cards, no multi-board
- Go 1.24 backend, CGO-free modernc.org/sqlite, embedded SPA, REST /api
- Preact + Vite + TS frontend, native HTML5 drag-and-drop, optimistic UI, dark Haven theme
- PVC (nfs-client) for SQLite, root container on NFS, simple nginx ingress havenllo.haven
- Dockerfile multi-arch build, Gitea CI via pipeline-actions@main

internal/domain/models.go

@@ -0,0 +1,42 @@
+package domain
+
+import "errors"
+
+var (
+	ErrNotFound = errors.New("resource not found")
+	ErrLastList = errors.New("the last remaining list cannot be deleted")
+)
+
+// ValidationError represents a user-correctable domain validation failure.
+type ValidationError struct{ Message string }
+
+func (e ValidationError) Error() string { return e.Message }
+
+type List struct {
+	ID        int64   `json:"id"`
+	Name      string  `json:"name"`
+	Position  float64 `json:"position"`
+	CardCount int     `json:"card_count,omitempty"`
+}
+
+type Card struct {
+	ID          int64   `json:"id"`
+	ListID      int64   `json:"list_id"`
+	Title       string  `json:"title"`
+	Description string  `json:"description"`
+	Position    float64 `json:"position"`
+	Done        bool    `json:"done"`
+	CreatedAt   string  `json:"created_at"`
+	UpdatedAt   string  `json:"updated_at"`
+}
+
+type ListWithCards struct {
+	ID       int64   `json:"id"`
+	Name     string  `json:"name"`
+	Position float64 `json:"position"`
+	Cards    []Card  `json:"cards"`
+}
+
+type Snapshot struct {
+	Lists []ListWithCards `json:"lists"`
+}