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/go.mod

@@ -3,6 +3,7 @@ module krafttrainer
 go 1.26.1
 
 require (
-	github.com/golang-migrate/migrate/v4 v4.19.1 // indirect
-	github.com/mattn/go-sqlite3 v1.14.37 // indirect
+	github.com/golang-migrate/migrate/v4 v4.19.1
+	github.com/google/uuid v1.6.0
+	github.com/mattn/go-sqlite3 v1.14.37
 )