chore: add README, Makefile, CI, lint config and --version

The repo had no README, Makefile, CI, linter config or version string.

- README.md: requirements (ffmpeg/ffprobe, plus yt-dlp for SoundCloud), build,
  the per-source credentials table, usage, the quality scale and the download
  database's migration behaviour. Every claim was checked against the code
  rather than written from memory, which is how the quality-range bug below was
  found. It also records the two deferred refactors (no shared HTTP layer, and
  metadata as map[string]any) so they are documented rather than lore, and
  notes that there is no LICENSE file.
- Makefile: build, install, test, race, cover, fmt, fmt-check, vet, lint,
  check, clean, help. `make build` stamps version/commit/date via -ldflags.
- .github/workflows/ci.yml: gofmt, vet, build, test and test -race, plus a
  lint job. The linter is intentionally unpinned, with a comment: a
  golangci-lint built against an older Go cannot analyse a go1.25 module, which
  is the same reason a locally installed staticcheck built with go1.24 cannot
  run here.
- .golangci.yml: standard set plus bodyclose, errorlint, nilerr and
  govet/copylocks. copylocks matters now that config.Config carries a mutex;
  bodyclose and errorlint target defects this codebase actually has.
- `rip --version` / `-V`, documented in the help output, falling back to the
  VCS revision the toolchain embeds for `go install` builds.

Fixes a bug introduced in the previous commit: Validate capped deezer quality
at 2 and soundcloud at 1, but config.toml.example documents deezer as 2/3/4 and
the global -q flag accepts 0-4, so a documented config would have failed to
load. deezer's buildFormatPriority treats anything >= 2 as FLAC and soundcloud
ignores the value entirely, so the real ceiling is 4 everywhere; only the Qobuz
and Beatport floor of 1 is a genuine constraint. Pinned with a test that walks
the whole documented range.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Makefile

@@ -0,0 +1,63 @@
+BINARY  := rip
+PKG     := ./cmd/rip
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
+COMMIT  ?= $(shell git rev-parse HEAD 2>/dev/null)
+DATE    ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
+
+LDFLAGS := -X main.version=$(VERSION) -X main.commit=$(COMMIT) -X main.date=$(DATE)
+
+.DEFAULT_GOAL := build
+
+.PHONY: build
+build: ## Build the rip binary
+	go build -ldflags "$(LDFLAGS)" -o $(BINARY) $(PKG)
+
+.PHONY: install
+install: ## Install rip into GOBIN
+	go install -ldflags "$(LDFLAGS)" $(PKG)
+
+.PHONY: test
+test: ## Run the test suite
+	go test ./...
+
+.PHONY: race
+race: ## Run the test suite under the race detector
+	go test -race ./...
+
+.PHONY: cover
+cover: ## Run tests and report per-package coverage
+	go test -coverprofile=coverage.out ./...
+	go tool cover -func=coverage.out | tail -1
+
+.PHONY: fmt
+fmt: ## Format all Go files
+	gofmt -w .
+
+.PHONY: fmt-check
+fmt-check: ## Fail if any file is not gofmt-clean
+	@out="$$(gofmt -l .)"; \
+	if [ -n "$$out" ]; then echo "not gofmt-clean:"; echo "$$out"; exit 1; fi
+
+.PHONY: vet
+vet: ## Run go vet
+	go vet ./...
+
+.PHONY: lint
+lint: ## Run golangci-lint if it is installed
+	@if command -v golangci-lint >/dev/null 2>&1; then \
+		golangci-lint run; \
+	else \
+		echo "golangci-lint not installed; skipping"; \
+		echo "  go install github.com/golangci/golangci-lint/cmd/golangci-lint@latest"; \
+	fi
+
+.PHONY: check
+check: fmt-check vet test ## Everything CI runs
+
+.PHONY: clean
+clean: ## Remove build and coverage artifacts
+	rm -f $(BINARY) coverage.out
+
+.PHONY: help
+help: ## List targets
+	@grep -hE '^[a-z-]+:.*?## ' $(MAKEFILE_LIST) | awk -F':.*?## ' '{printf "  %-12s %s\n", $$1, $$2}'