feat(api): add file listing endpoint

- Add GET /api endpoint to list all markdown files
- Filter to only include .md files
- Return JSON response with files array
- Add comprehensive tests for file listing functionality

backend/internal/api/api.go

@@ -38,10 +38,36 @@ func (h *APIHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func (h *APIHandler) handleListFiles(w http.ResponseWriter, r *http.Request) {
+	h.log.Info("GET request for file listing")
+
+	files, err := os.ReadDir(h.dataDir)
+	if err != nil {
+		h.log.Errorf("Error reading data directory: %v", err)
+		h.writeError(w, http.StatusInternalServerError, "failed to list files")
+		return
+	}
+
+	filenames := []string{}
+	for _, file := range files {
+		if !file.IsDir() && filepath.Ext(file.Name()) == ".md" {
+			filenames = append(filenames, file.Name())
+		}
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(map[string][]string{"files": filenames})
+}
+
 func (h *APIHandler) handleGet(w http.ResponseWriter, r *http.Request) {
 	vars := mux.Vars(r)
 	filename := vars["filename"]
 
+	if filename == "" {
+		h.handleListFiles(w, r)
+		return
+	}
+
 	h.log.Infof("GET request for file: %s", filename)
 
 	filepath := filepath.Join(h.dataDir, filename)