Replace per-kind reclassify with a single global reclassify that respects manual and Race locks

Manual assignments are the user's definitive word and Race assignments come
from a hard Garmin fact (eventType.typeKey), not a retunable rule -- neither
is ever touched by reclassify again, and Race can no longer be set by hand
via the review queue. Adds an Activities page so this locked/unlocked status
is visible per workout, since Review Queue only ever showed unresolved items.

backend/internal/api/activities.go

@@ -9,6 +9,21 @@ import (
 	"smartrun/backend/internal/store"
 )
 
+// activityListItem is one row of the activities list, enriched with its
+// current classification so the frontend can show what kind it's assigned
+// to and whether that assignment is locked (see Locked's doc comment).
+type activityListItem struct {
+	store.Activity
+	WorkoutKindID    *int64  `json:"workout_kind_id"`
+	WorkoutKindName  *string `json:"workout_kind_name"`
+	AssignmentSource *string `json:"assignment_source"`
+	AssignmentStatus *string `json:"assignment_status"`
+	// Locked is true when a global reclassify pass will never touch this
+	// activity again: a manual assignment is the user's definitive word, and
+	// a Race assignment comes from a hard Garmin fact, not a retunable rule.
+	Locked bool `json:"locked"`
+}
+
 func (s *Server) handleListActivities(w http.ResponseWriter, r *http.Request) {
 	q := r.URL.Query()
 	filter := store.ActivityFilter{
@@ -27,7 +42,40 @@ func (s *Server) handleListActivities(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
-	writeJSON(w, http.StatusOK, activities)
+
+	kinds, err := s.DB.ListWorkoutKinds(r.Context(), false)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+	kindNames := make(map[int64]string, len(kinds))
+	for _, k := range kinds {
+		kindNames[k.ID] = k.Name
+	}
+
+	resp := make([]activityListItem, 0, len(activities))
+	for _, a := range activities {
+		item := activityListItem{Activity: a}
+		assignment, ok, err := s.DB.CurrentAssignment(r.Context(), a.ID)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, err.Error())
+			return
+		}
+		if ok {
+			source, status := assignment.AssignmentSource, assignment.Status
+			item.AssignmentSource, item.AssignmentStatus = &source, &status
+			if assignment.WorkoutKindID != nil {
+				item.WorkoutKindID = assignment.WorkoutKindID
+				if name, found := kindNames[*assignment.WorkoutKindID]; found {
+					item.WorkoutKindName = &name
+				}
+			}
+			item.Locked = source == store.AssignmentSourceManual ||
+				(item.WorkoutKindName != nil && *item.WorkoutKindName == "Race")
+		}
+		resp = append(resp, item)
+	}
+	writeJSON(w, http.StatusOK, resp)
 }
 
 func (s *Server) handleGetActivity(w http.ResponseWriter, r *http.Request) {