Add feature toggles system with settings UI (#74)

- Add feature_toggles table (migration 012)
- Add source_config table for future source selection (migration 013)
- Create settings page at /settings with:
  - Feature toggle management (enable/disable/create/delete)
  - Data source configuration (sync and toggle boards/calendars)
- Add store methods for feature toggles and source config
- Add GetCalendarList and GetTaskLists to Google API clients
- Document feature toggle workflow in DESIGN.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

2 files changed, 236 insertions, 0 deletions

diff --git a/internal/handlers/settings.go b/internal/handlers/settings.go
new file mode 100644
index 0000000..1eabdf5
--- /dev/null
+++ b/internal/handlers/settings.go
@@ -0,0 +1,232 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"task-dashboard/internal/models"
+)
+
+// HandleSettingsPage renders the settings page
+func (h *Handler) HandleSettingsPage(w http.ResponseWriter, r *http.Request) {
+	configs, _ := h.store.GetSourceConfigs()
+	toggles, _ := h.store.GetFeatureToggles()
+
+	// Group configs by source
+	bySource := make(map[string][]models.SourceConfig)
+	for _, cfg := range configs {
+		bySource[cfg.Source] = append(bySource[cfg.Source], cfg)
+	}
+
+	data := struct {
+		Configs  map[string][]models.SourceConfig
+		Sources  []string
+		Toggles  []models.FeatureToggle
+	}{
+		Configs: bySource,
+		Sources: []string{"trello", "todoist", "gcal", "gtasks"},
+		Toggles: toggles,
+	}
+
+	if err := h.templates.ExecuteTemplate(w, "settings.html", data); err != nil {
+		JSONError(w, http.StatusInternalServerError, "Failed to render settings", err)
+	}
+}
+
+// HandleSyncSources fetches available items from all sources and syncs to config
+func (h *Handler) HandleSyncSources(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	// Sync Trello boards
+	if h.trelloClient != nil {
+		boards, err := h.trelloClient.GetBoards(ctx)
+		if err == nil {
+			var items []models.SourceConfig
+			for _, b := range boards {
+				items = append(items, models.SourceConfig{
+					Source:   "trello",
+					ItemType: "board",
+					ItemID:   b.ID,
+					ItemName: b.Name,
+				})
+			}
+			_ = h.store.SyncSourceConfigs("trello", "board", items)
+		}
+	}
+
+	// Sync Todoist projects
+	if h.todoistClient != nil {
+		projects, err := h.todoistClient.GetProjects(ctx)
+		if err == nil {
+			var items []models.SourceConfig
+			for _, p := range projects {
+				items = append(items, models.SourceConfig{
+					Source:   "todoist",
+					ItemType: "project",
+					ItemID:   p.ID,
+					ItemName: p.Name,
+				})
+			}
+			_ = h.store.SyncSourceConfigs("todoist", "project", items)
+		}
+	}
+
+	// Sync Google Calendar calendars
+	if h.googleCalendarClient != nil {
+		calendars, err := h.googleCalendarClient.GetCalendarList(ctx)
+		if err == nil {
+			var items []models.SourceConfig
+			for _, c := range calendars {
+				items = append(items, models.SourceConfig{
+					Source:   "gcal",
+					ItemType: "calendar",
+					ItemID:   c.ID,
+					ItemName: c.Name,
+				})
+			}
+			_ = h.store.SyncSourceConfigs("gcal", "calendar", items)
+		}
+	}
+
+	// Sync Google Tasks lists
+	if h.googleTasksClient != nil {
+		lists, err := h.googleTasksClient.GetTaskLists(ctx)
+		if err == nil {
+			var items []models.SourceConfig
+			for _, l := range lists {
+				items = append(items, models.SourceConfig{
+					Source:   "gtasks",
+					ItemType: "tasklist",
+					ItemID:   l.ID,
+					ItemName: l.Name,
+				})
+			}
+			_ = h.store.SyncSourceConfigs("gtasks", "tasklist", items)
+		}
+	}
+
+	// Return updated configs
+	h.HandleSettingsPage(w, r)
+}
+
+// HandleToggleSourceConfig toggles a source config item
+func (h *Handler) HandleToggleSourceConfig(w http.ResponseWriter, r *http.Request) {
+	if err := r.ParseForm(); err != nil {
+		JSONError(w, http.StatusBadRequest, "Failed to parse form", err)
+		return
+	}
+
+	source := r.FormValue("source")
+	itemType := r.FormValue("item_type")
+	itemID := r.FormValue("item_id")
+	enabled := r.FormValue("enabled") == "true"
+
+	if source == "" || itemType == "" || itemID == "" {
+		JSONError(w, http.StatusBadRequest, "Missing required fields", nil)
+		return
+	}
+
+	if err := h.store.SetSourceConfigEnabled(source, itemType, itemID, enabled); err != nil {
+		JSONError(w, http.StatusInternalServerError, "Failed to update config", err)
+		return
+	}
+
+	// Invalidate relevant cache
+	switch source {
+	case "trello":
+		_ = h.store.InvalidateCache("trello_boards")
+	case "todoist":
+		_ = h.store.InvalidateCache("todoist_tasks")
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(map[string]bool{"enabled": enabled})
+}
+
+// HandleGetSourceOptions returns available options for a source (HTMX partial)
+func (h *Handler) HandleGetSourceOptions(w http.ResponseWriter, r *http.Request) {
+	source := chi.URLParam(r, "source")
+	if source == "" {
+		JSONError(w, http.StatusBadRequest, "Source required", nil)
+		return
+	}
+
+	configs, err := h.store.GetSourceConfigsBySource(source)
+	if err != nil {
+		JSONError(w, http.StatusInternalServerError, "Failed to load configs", err)
+		return
+	}
+
+	data := struct {
+		Source  string
+		Configs []models.SourceConfig
+	}{source, configs}
+
+	HTMLResponse(w, h.templates, "settings-source-options", data)
+}
+
+// HandleToggleFeature toggles a feature flag
+func (h *Handler) HandleToggleFeature(w http.ResponseWriter, r *http.Request) {
+	if err := r.ParseForm(); err != nil {
+		JSONError(w, http.StatusBadRequest, "Failed to parse form", err)
+		return
+	}
+
+	name := r.FormValue("name")
+	enabled := r.FormValue("enabled") == "true"
+
+	if name == "" {
+		JSONError(w, http.StatusBadRequest, "Feature name required", nil)
+		return
+	}
+
+	if err := h.store.SetFeatureEnabled(name, enabled); err != nil {
+		JSONError(w, http.StatusInternalServerError, "Failed to update feature", err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(map[string]bool{"enabled": enabled})
+}
+
+// HandleCreateFeature creates a new feature toggle
+func (h *Handler) HandleCreateFeature(w http.ResponseWriter, r *http.Request) {
+	if err := r.ParseForm(); err != nil {
+		JSONError(w, http.StatusBadRequest, "Failed to parse form", err)
+		return
+	}
+
+	name := r.FormValue("name")
+	description := r.FormValue("description")
+
+	if name == "" {
+		JSONError(w, http.StatusBadRequest, "Feature name required", nil)
+		return
+	}
+
+	if err := h.store.CreateFeatureToggle(name, description, false); err != nil {
+		JSONError(w, http.StatusInternalServerError, "Failed to create feature", err)
+		return
+	}
+
+	// Return updated toggles list
+	h.HandleSettingsPage(w, r)
+}
+
+// HandleDeleteFeature removes a feature toggle
+func (h *Handler) HandleDeleteFeature(w http.ResponseWriter, r *http.Request) {
+	name := chi.URLParam(r, "name")
+	if name == "" {
+		JSONError(w, http.StatusBadRequest, "Feature name required", nil)
+		return
+	}
+
+	if err := h.store.DeleteFeatureToggle(name); err != nil {
+		JSONError(w, http.StatusInternalServerError, "Failed to delete feature", err)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+}
diff --git a/internal/handlers/timeline_logic_test.go b/internal/handlers/timeline_logic_test.go
index 5d0a425..5fe995f 100644
--- a/internal/handlers/timeline_logic_test.go
+++ b/internal/handlers/timeline_logic_test.go
@@ -27,6 +27,10 @@ func (m *MockCalendarClient) GetEventsByDateRange(ctx context.Context, start, en
 	return m.Events, m.Err
 }
 
+func (m *MockCalendarClient) GetCalendarList(ctx context.Context) ([]models.CalendarInfo, error) {
+	return nil, m.Err
+}
+
 func setupTestStore(t *testing.T) *store.Store {
 	t.Helper()
 	tempDir := t.TempDir()