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/store/image_store.go

@@ -0,0 +1,74 @@
+package store
+
+import (
+	"fmt"
+	"krafttrainer/internal/model"
+)
+
+// ListExerciseImages gibt alle Bilder einer Übung zurück.
+func (s *Store) ListExerciseImages(exerciseID int64) ([]model.ExerciseImage, error) {
+	rows, err := s.db.Query(`
+		SELECT id, exercise_id, filename, sort_order, created_at
+		FROM exercise_images
+		WHERE exercise_id = ?
+		ORDER BY sort_order, id`, exerciseID,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("Bilder abfragen: %w", err)
+	}
+	defer rows.Close()
+
+	var images []model.ExerciseImage
+	for rows.Next() {
+		var img model.ExerciseImage
+		if err := rows.Scan(&img.ID, &img.ExerciseID, &img.Filename, &img.SortOrder, &img.CreatedAt); err != nil {
+			return nil, fmt.Errorf("Bild scannen: %w", err)
+		}
+		images = append(images, img)
+	}
+	if images == nil {
+		images = []model.ExerciseImage{}
+	}
+	return images, rows.Err()
+}
+
+// CreateExerciseImage speichert einen Bild-Eintrag.
+func (s *Store) CreateExerciseImage(exerciseID int64, filename string) (*model.ExerciseImage, error) {
+	// sort_order = bisherige Anzahl
+	var count int
+	s.db.QueryRow(`SELECT COUNT(*) FROM exercise_images WHERE exercise_id = ?`, exerciseID).Scan(&count)
+
+	result, err := s.db.Exec(`
+		INSERT INTO exercise_images (exercise_id, filename, sort_order)
+		VALUES (?, ?, ?)`, exerciseID, filename, count,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("Bild speichern: %w", err)
+	}
+
+	id, _ := result.LastInsertId()
+	var img model.ExerciseImage
+	err = s.db.QueryRow(`
+		SELECT id, exercise_id, filename, sort_order, created_at
+		FROM exercise_images WHERE id = ?`, id,
+	).Scan(&img.ID, &img.ExerciseID, &img.Filename, &img.SortOrder, &img.CreatedAt)
+	if err != nil {
+		return nil, fmt.Errorf("Bild abfragen: %w", err)
+	}
+	return &img, nil
+}
+
+// DeleteExerciseImage löscht ein Bild und gibt den Dateinamen zurück.
+func (s *Store) DeleteExerciseImage(imageID int64) (string, error) {
+	var filename string
+	err := s.db.QueryRow(`SELECT filename FROM exercise_images WHERE id = ?`, imageID).Scan(&filename)
+	if err != nil {
+		return "", fmt.Errorf("Bild nicht gefunden: %w", err)
+	}
+
+	_, err = s.db.Exec(`DELETE FROM exercise_images WHERE id = ?`, imageID)
+	if err != nil {
+		return "", fmt.Errorf("Bild löschen: %w", err)
+	}
+	return filename, nil
+}