Add configurable report module composition

internal/report/period_test.go

@@ -345,6 +345,47 @@ func TestRegistryDefinitionsDeclareDefaultModules(t *testing.T) {
 	}
 }
 
+func TestRegistryAppliesModuleOverridesWithoutChangingDefaults(t *testing.T) {
+	base := DefaultRegistry()
+	overridden, err := base.WithModuleOverrides(map[ID][]module.ConfigItem{
+		DailyToday: {
+			{ID: module.Metadata},
+			{ID: module.AlertDigest},
+		},
+	})
+	if err != nil {
+		t.Fatalf("WithModuleOverrides() error = %v", err)
+	}
+
+	definition, err := overridden.Lookup(DailyToday)
+	if err != nil {
+		t.Fatalf("Lookup(overridden) error = %v", err)
+	}
+	if !reflect.DeepEqual(definition.ModuleIDs(), []module.ID{module.Metadata, module.AlertDigest}) {
+		t.Fatalf("overridden ModuleIDs() = %#v, want metadata and alert digest", definition.ModuleIDs())
+	}
+
+	defaultDefinition, err := base.Lookup(DailyToday)
+	if err != nil {
+		t.Fatalf("Lookup(default) error = %v", err)
+	}
+	if len(defaultDefinition.ModuleIDs()) <= len(definition.ModuleIDs()) {
+		t.Fatalf("default ModuleIDs() = %#v, want original defaults unchanged", defaultDefinition.ModuleIDs())
+	}
+}
+
+func TestRegistryRejectsModuleOverrideForUnknownReport(t *testing.T) {
+	_, err := DefaultRegistry().WithModuleOverrides(map[ID][]module.ConfigItem{
+		ID("unknown"): {{ID: module.Metadata}},
+	})
+	if err == nil {
+		t.Fatal("WithModuleOverrides() error = nil, want unknown report")
+	}
+	if !strings.Contains(err.Error(), `unknown report "unknown"`) {
+		t.Fatalf("error = %q, want unknown report context", err.Error())
+	}
+}
+
 func TestResolvedMetadata(t *testing.T) {
 	location := mustLoadLocation(t)
 	resolved, err := Resolve(DailyToday, ResolveRequest{Now: mustParse("2026-05-29T05:00:00-05:00"), Location: location})

internal/report/registry.go

@@ -84,6 +84,23 @@ func DefaultRegistry() Registry {
 	return registry
 }
 
+func (r Registry) WithModuleOverrides(overrides map[ID][]module.ConfigItem) (Registry, error) {
+	next := Registry{definitions: map[ID]Definition{}}
+	for id, definition := range r.definitions {
+		definition.Modules = append([]module.ConfigItem(nil), definition.Modules...)
+		next.definitions[id] = definition
+	}
+	for id, items := range overrides {
+		definition, ok := next.definitions[id]
+		if !ok {
+			return Registry{}, fmt.Errorf("unknown report %q", id)
+		}
+		definition.Modules = cloneModuleItems(items)
+		next.definitions[id] = definition
+	}
+	return next, nil
+}
+
 func dailyTodayModules() []module.ConfigItem {
 	return moduleItems(
 		module.Metadata,
@@ -154,6 +171,12 @@ func moduleItems(ids ...module.ID) []module.ConfigItem {
 	return items
 }
 
+func cloneModuleItems(items []module.ConfigItem) []module.ConfigItem {
+	cloned := make([]module.ConfigItem, len(items))
+	copy(cloned, items)
+	return cloned
+}
+
 func (r Registry) Lookup(id ID) (Definition, error) {
 	definition, ok := r.definitions[id]
 	if !ok {