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/store_test.go

@@ -96,7 +96,7 @@ func TestKindAssignment_AppendOnlyHistoryAndCurrentView(t *testing.T) {
 	}
 
 	kindID, err := db.CreateWorkoutKind(ctx, WorkoutKind{
-		Name:     "Tempo",
+		Name:     "Test Assignment Custom",
 		RuleJSON: `{"match":"all","conditions":[]}`,
 		IsActive: true,
 	})