feat: Add REST API handlers for exercises, plans, and sessions

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

backend/internal/api/handlers.go

@@ -0,0 +1,36 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// Response helpers
+
+func respondJSON(w http.ResponseWriter, status int, data interface{}) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if data != nil {
+		json.NewEncoder(w).Encode(data)
+	}
+}
+
+func respondError(w http.ResponseWriter, status int, message string) {
+	respondJSON(w, status, map[string]string{"error": message})
+}
+
+func parseID(r *http.Request, prefix string) (int64, error) {
+	path := strings.TrimPrefix(r.URL.Path, prefix)
+	path = strings.TrimSuffix(path, "/")
+	parts := strings.Split(path, "/")
+	if len(parts) == 0 {
+		return 0, nil
+	}
+	return strconv.ParseInt(parts[0], 10, 64)
+}
+
+func decodeJSON(r *http.Request, v interface{}) error {
+	return json.NewDecoder(r.Body).Decode(v)
+}