Add exercise numbers, image uploads, version display, session resume, and training sparklines

- Exercise number (UF#): optional field on exercises, displayed in cards, training, and sets
- Import training plan numbers via migration 005 (UPDATE by name)
- Exercise images: JPG upload with multi-image support per exercise (migration 006)
- Version endpoint (GET /api/v1/version) with ldflags injection in Makefile and Dockerfile
- Version displayed on settings page
- Session resume: GET /api/v1/sessions/active endpoint, auto-resume on training page load
- Block new session while one is active (409 Conflict)
- e1RM sparkline chart per exercise during training (Epley formula)
- Fix CORS: add X-User-ID to allowed headers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/handler.go

@@ -38,6 +38,7 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("DELETE /api/v1/sets/{id}", h.handleDeleteSet)
 
 	// Sessions
+	mux.HandleFunc("GET /api/v1/sessions/active", h.handleGetActiveSession)
 	mux.HandleFunc("POST /api/v1/sessions", h.handleCreateSession)
 	mux.HandleFunc("GET /api/v1/sessions", h.handleListSessions)
 	mux.HandleFunc("GET /api/v1/sessions/{id}", h.handleGetSession)