chore(spa-server): add Makefile and OCI image labels to Dockerfile

Makefile:
- dry-run target (default) shows registry, branch, SHA, version, all computed
  image tags, and build args — no side effects
- build/push/release targets wire docker build and docker push
- Branch-aware tag strategy:
    main        → <version> + latest
    develop     → <version>-dev-<short-sha>
    release/*   → <version>-qa-<short-sha>
- Version resolved from the highest semver tag at HEAD; falls back to the most
  recent reachable tag via git describe
- BASE_DIGEST resolved at build time from the local Docker daemon; dry-run
  prints a hint when the base image is not yet pulled
- Note: case/esac cannot be used inside $(shell) — Make's parser matches the
  pattern ")" as the function's closing paren before the shell sees it;
  if/elif/else/fi used instead

Dockerfile:
- OCI standard labels added to the final stage via build args (VERSION, GIT_SHA,
  BASE_DIGEST):
    org.opencontainers.image.source    — repo URL
    org.opencontainers.image.version   — semver tag
    org.opencontainers.image.revision  — git commit SHA
    org.opencontainers.image.base.name — alpine:3.21
    org.opencontainers.image.base.digest — digest of base image at build time
- Custom label:
    dev.nochebuena.healthz — /health

Makefile

@@ -0,0 +1,89 @@
+# ── Configuration ─────────────────────────────────────────────────────────────
+
+REGISTRY   := code.nochebuena.dev/einherjar/spa-server
+BASE_IMAGE := alpine:3.21
+
+# ── Git metadata ──────────────────────────────────────────────────────────────
+
+BRANCH := $(shell git rev-parse --abbrev-ref HEAD 2>/dev/null || echo unknown)
+SHA    := $(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
+
+# Highest semver tag pointing at HEAD; falls back to the most recent reachable tag.
+_TAG_AT_HEAD := $(shell git tag --points-at HEAD 2>/dev/null | sort -V | tail -1)
+VERSION      := $(if $(_TAG_AT_HEAD),$(_TAG_AT_HEAD),$(shell git describe --tags --abbrev=0 2>/dev/null || echo v0.0.0))
+
+# ── Branch suffix ─────────────────────────────────────────────────────────────
+#   main       →  (none)
+#   develop    →  -dev-<sha>
+#   release/*  →  -qa-<sha>
+#   other      →  -dev-<sha>
+#
+# Note: case/esac cannot be used inside $(shell) — the pattern delimiters ")"
+# are mistaken by Make's parser for the closing paren of the function call.
+# if/elif is used instead.
+
+BRANCH_SUFFIX := $(shell \
+	b='$(BRANCH)'; s='$(SHA)'; \
+	if [ "$$b" = "main" ]; then \
+		echo ""; \
+	elif [ "$$b" = "develop" ]; then \
+		echo "-dev-$$s"; \
+	elif echo "$$b" | grep -q "^release/"; then \
+		echo "-qa-$$s"; \
+	else \
+		echo "-dev-$$s"; \
+	fi)
+
+# ── Image tags ────────────────────────────────────────────────────────────────
+
+VERSIONED_TAG := $(REGISTRY):$(VERSION)$(BRANCH_SUFFIX)
+LATEST_TAG    := $(if $(filter main,$(BRANCH)),$(REGISTRY):latest,)
+ALL_TAGS      := $(strip $(VERSIONED_TAG) $(LATEST_TAG))
+TAG_FLAGS     := $(foreach t,$(ALL_TAGS),--tag $(t) )
+
+# ── Targets ───────────────────────────────────────────────────────────────────
+
+.DEFAULT_GOAL := dry-run
+
+.PHONY: dry-run build push release help
+
+## dry-run : show what would be built and pushed — no side effects.
+dry-run:
+	@echo ""
+	@echo "  Registry : $(REGISTRY)"
+	@echo "  Branch   : $(BRANCH)"
+	@echo "  SHA      : $(SHA)"
+	@echo "  Version  : $(VERSION)"
+	@echo ""
+	@echo "  Tags:"
+	@$(foreach t,$(ALL_TAGS),echo "    $(t)";)
+	@echo ""
+	@echo "  Build args:"
+	@echo "    VERSION     = $(VERSION)"
+	@echo "    GIT_SHA     = $(SHA)"
+	@BASE=$$(docker inspect --format='{{index .RepoDigests 0}}' $(BASE_IMAGE) 2>/dev/null | head -1); \
+		echo "    BASE_DIGEST = $${BASE:-(not resolved — run: docker pull $(BASE_IMAGE))}"
+	@echo ""
+
+## build    : build the image with all computed tags and OCI labels.
+build:
+	docker build \
+		$(TAG_FLAGS) \
+		--build-arg VERSION=$(VERSION) \
+		--build-arg GIT_SHA=$(SHA) \
+		--build-arg BASE_DIGEST=$$(docker inspect --format='{{index .RepoDigests 0}}' $(BASE_IMAGE) 2>/dev/null | head -1) \
+		.
+
+## push     : push all computed tags to the registry.
+push:
+	@for tag in $(ALL_TAGS); do \
+		echo "→ pushing $$tag"; \
+		docker push $$tag || exit 1; \
+	done
+
+## release  : build then push.
+release: build push
+
+## help     : list available targets.
+help:
+	@grep -E '^## ' Makefile | sed 's/^## /  /'