fix: report actual version instead of hardcoded 3.0.0-dev

- Add version variable to main.go, set via -X ldflags at build time
- Pass version through to Server, used in the / endpoint response
- Makefile passes VERSION from git describe
- Dockerfile accepts VERSION build arg
- Woodpecker passes CI_COMMIT_TAG as VERSION

Makefile

@@ -12,7 +12,7 @@ check-go:
 	fi
 
 build: check-go tidy
-	go build -ldflags="-s -w" -o $(BINARY) ./cmd/artifactapi
+	go build -ldflags="-s -w -X main.version=$(VERSION)" -o $(BINARY) ./cmd/artifactapi
 
 test: check-go
 	go test -race -count=1 ./pkg/... ./internal/...