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

Dockerfile

@@ -9,7 +9,8 @@ RUN go mod download
 
 COPY . .
 
-RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o artifactapi ./cmd/artifactapi
+ARG VERSION=dev
+RUN CGO_ENABLED=0 go build -ldflags="-s -w -X main.version=${VERSION}" -o artifactapi ./cmd/artifactapi
 
 FROM gcr.io/distroless/static-debian12:nonroot