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.go

@@ -8,22 +8,24 @@ import (
 
 // Exercise repräsentiert eine Kraftübung.
 type Exercise struct {
-	ID          int64      `json:"id"`
-	Name        string     `json:"name"`
-	Description string     `json:"description"`
-	MuscleGroup string     `json:"muscle_group"`
-	WeightStepKg float64   `json:"weight_step_kg"`
-	CreatedAt   time.Time  `json:"created_at"`
-	UpdatedAt   time.Time  `json:"updated_at"`
-	DeletedAt   *time.Time `json:"deleted_at,omitempty"`
+	ID             int64      `json:"id"`
+	Name           string     `json:"name"`
+	Description    string     `json:"description"`
+	MuscleGroup    string     `json:"muscle_group"`
+	WeightStepKg   float64    `json:"weight_step_kg"`
+	ExerciseNumber *int       `json:"exercise_number,omitempty"`
+	CreatedAt      time.Time  `json:"created_at"`
+	UpdatedAt      time.Time  `json:"updated_at"`
+	DeletedAt      *time.Time `json:"deleted_at,omitempty"`
 }
 
 // CreateExerciseRequest enthält die Felder zum Anlegen einer Übung.
 type CreateExerciseRequest struct {
-	Name         string   `json:"name"`
-	Description  string   `json:"description"`
-	MuscleGroup  string   `json:"muscle_group"`
-	WeightStepKg *float64 `json:"weight_step_kg"`
+	Name           string   `json:"name"`
+	Description    string   `json:"description"`
+	MuscleGroup    string   `json:"muscle_group"`
+	WeightStepKg   *float64 `json:"weight_step_kg"`
+	ExerciseNumber *int     `json:"exercise_number,omitempty"`
 }
 
 // Validate prüft und normalisiert den Request. Setzt Default für WeightStepKg.