feat: reseed workout_kinds with the fixed 7-type running taxonomy

- Add migration 0004_workout_taxonomy.sql to reset workout_kinds table
  with exactly 7 fixed named kinds (Easy Run, Long Run, Threshold 30',
  Threshold 60', Tempo, Interval, MAS Test), each with a never-matching
  placeholder rule
- Add test TestWorkoutTaxonomy_SeededWithSevenFixedTypes to verify all
  7 kinds are seeded and active
- Update TestKindAssignment_AppendOnlyHistoryAndCurrentView to use a
  unique kind name to avoid conflict with seeded kinds

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/store/workoutkinds_taxonomy_test.go

@@ -0,0 +1,36 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestWorkoutTaxonomy_SeededWithSevenFixedTypes(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	kinds, err := db.ListWorkoutKinds(ctx, true)
+	if err != nil {
+		t.Fatalf("ListWorkoutKinds: %v", err)
+	}
+	if len(kinds) != 7 {
+		t.Fatalf("expected 7 seeded workout kinds, got %d: %+v", len(kinds), kinds)
+	}
+
+	wantNames := map[string]bool{
+		"Easy Run": false, "Long Run": false, "Threshold 30'": false, "Threshold 60'": false,
+		"Tempo": false, "Interval": false, "MAS Test": false,
+	}
+	for _, k := range kinds {
+		if _, ok := wantNames[k.Name]; !ok {
+			t.Errorf("unexpected seeded kind name %q", k.Name)
+			continue
+		}
+		wantNames[k.Name] = true
+	}
+	for name, found := range wantNames {
+		if !found {
+			t.Errorf("expected seeded kind %q, not found", name)
+		}
+	}
+}