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/model/exercise_image.go

@@ -0,0 +1,12 @@
+package model
+
+import "time"
+
+// ExerciseImage repräsentiert ein Bild einer Übung.
+type ExerciseImage struct {
+	ID         int64     `json:"id"`
+	ExerciseID int64     `json:"exercise_id"`
+	Filename   string    `json:"filename"`
+	SortOrder  int       `json:"sort_order"`
+	CreatedAt  time.Time `json:"created_at"`
+}