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>

frontend/src/components/exercises/ExerciseCard.tsx

@@ -15,7 +15,12 @@ export function ExerciseCard({ exercise, onEdit, onDelete }: ExerciseCardProps)
     <div className="bg-gray-900 border border-gray-800 rounded-xl p-4">
       <div className="flex items-start justify-between gap-2">
         <div className="flex-1 min-w-0">
-          <h3 className="font-semibold text-gray-100 truncate">{exercise.name}</h3>
+          <h3 className="font-semibold text-gray-100 truncate">
+            {exercise.exercise_number != null && (
+              <span className="text-blue-400 mr-1.5">#{exercise.exercise_number}</span>
+            )}
+            {exercise.name}
+          </h3>
           {exercise.description && (
             <p className="text-sm text-gray-400 mt-1">{exercise.description}</p>
           )}