Implement time off management (Issue #3)

Add full time-off lifecycle: create/edit/delete with shift conflict
detection, auto-removal from conflicting shifts with admin notification,
shift restoration on admin delete, and hard block on assigning volunteers
with approved time off to shifts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/schedule/handler.go

@@ -19,9 +19,15 @@ type Notifier interface {
 	CreateNotification(ctx context.Context, volunteerID int64, message string) error
 }
 
+// TimeOffChecker checks whether a volunteer has approved time off on a date (FR-T06).
+type TimeOffChecker interface {
+	HasApprovedTimeOff(ctx context.Context, volunteerID int64, date string) (bool, error)
+}
+
 type Handler struct {
-	store    Storer
-	notifier Notifier
+	store          Storer
+	notifier       Notifier
+	timeOffChecker TimeOffChecker
 }
 
 // Storer is the interface the Handler depends on.
@@ -41,13 +47,13 @@ type Storer interface {
 	ConfirmShift(ctx context.Context, instanceID, volunteerID int64) error
 }
 
-func NewHandler(store *Store, notifier Notifier) *Handler {
-	return &Handler{store: store, notifier: notifier}
+func NewHandler(store *Store, notifier Notifier, timeOffChecker TimeOffChecker) *Handler {
+	return &Handler{store: store, notifier: notifier, timeOffChecker: timeOffChecker}
 }
 
 // NewHandlerFromInterfaces constructs a Handler from interface values, intended for testing.
-func NewHandlerFromInterfaces(store Storer, notifier Notifier) *Handler {
-	return &Handler{store: store, notifier: notifier}
+func NewHandlerFromInterfaces(store Storer, notifier Notifier, timeOffChecker TimeOffChecker) *Handler {
+	return &Handler{store: store, notifier: notifier, timeOffChecker: timeOffChecker}
 }
 
 // ---------------------------------------------------------------------------
@@ -263,6 +269,29 @@ func (h *Handler) UpdateInstance(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// FR-T06: Block assigning volunteers with approved time off on the shift date
+	if in.VolunteerIDs != nil && h.timeOffChecker != nil {
+		existing, getErr := h.store.GetInstance(r.Context(), id)
+		if getErr != nil && !errors.Is(getErr, ErrNotFound) {
+			respond.Error(w, http.StatusInternalServerError, "could not get shift")
+			return
+		}
+		if existing != nil {
+			for _, vid := range *in.VolunteerIDs {
+				hasTimeOff, checkErr := h.timeOffChecker.HasApprovedTimeOff(r.Context(), vid, existing.Date)
+				if checkErr != nil {
+					respond.Error(w, http.StatusInternalServerError, "could not check time off")
+					return
+				}
+				if hasTimeOff {
+					respond.Error(w, http.StatusConflict,
+						fmt.Sprintf("Volunteer %d has approved time off on %s. Remove the time off first.", vid, existing.Date))
+					return
+				}
+			}
+		}
+	}
+
 	inst, added, err := h.store.UpdateInstance(r.Context(), id, in)
 	if errors.Is(err, ErrNotFound) {
 		respond.Error(w, http.StatusNotFound, "shift not found")