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>

Makefile

@@ -1,3 +1,5 @@
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
+
 .PHONY: dev-backend dev-frontend build clean
 
 dev-backend:
@@ -10,7 +12,7 @@ build:
 	cd frontend && pnpm install && pnpm build
 	find backend/static -not -name 'embed.go' -not -name '.gitkeep' -not -path backend/static -delete
 	cp -r frontend/dist/* backend/static/
-	cd backend && GO111MODULE=on CGO_ENABLED=1 go build -o ../krafttrainer ./cmd/server
+	cd backend && GO111MODULE=on CGO_ENABLED=1 go build -ldflags "-X main.Version=$(VERSION)" -o ../krafttrainer ./cmd/server
 
 clean:
 	rm -f krafttrainer