Add github_alpine metadata-only package type (#115)

## Why

`github_alpine` is the Alpine/apk analog of the existing `github_deb`/`github_rpm` metadata-only remotes. It lets a plain GitHub-releases repo of `.apk` files be consumed as a real apk repository without artifactapi ever precaching the packages: it scans the repo's releases, derives each package's `.PKGINFO` from a ranged prefix fetch, synthesizes a per-arch `APKINDEX.tar.gz` from the cached metadata, and redirects the actual `.apk` downloads to a backend `releases_remote`. It stacks on the apk-local branch, reusing that work's alpine APKINDEX generator, `.apk`/`.PKGINFO` parser, Q1 pull-checksum, and `AlpineMetadata` store.

## How

- **`pkg/models`**: add `PackageGitHubAlpine` to the enum + validators (and test).
- **`internal/provider/alpine/github.go`**: the `github_alpine` provider. `ServeRemote` serves per-arch `<arch>/APKINDEX.tar.gz` (reusing `generateAPKIndex` over arch-filtered `AlpineMetadata` rows, `normalizeIndexPath` for apk's `./` dot-segment), 302-redirects `*.apk` to `{proxyBaseURL}/api/v1/remote/{releases_remote}/{path}`, and cold-starts with a 503 + `Retry-After`. `scanWithState` lists releases with ETag/If-None-Match and incrementally derives/prunes. `deriveAsset` does a **ranged GET of just the front of the `.apk`** — the control gzip stream carrying `.PKGINFO` sits near the front — doubling the range on truncation; it parses `.PKGINFO` and computes the `C:` Q1 checksum (`Q1`+base64(sha1(control stream))). `FilePath` = the github-relative asset path so the redirect resolves.
- **`internal/provider/alpine/syncer.go`**: a parallel background `Syncer` (own worker pool, shared rate limiter, deduped queue, DB lease), separate from the deb/rpm syncers.
- **`internal/database/alpine_github_sync.go`** + `github_alpine_sync_state` table: `ListGitHubAlpineRemotes` + Claim/Release per-remote sync lease, kept separate from the deb/rpm tables.
- **`internal/server/server.go`**: construct + `Run` the alpine syncer alongside deb/rpm and register it in the `PackageType→Primer` map (priming on create then flows through the existing generic `remotes.go` path).

The rpm/deb providers, syncers, and tables are untouched — this adds parallel alpine equivalents and reuses shared helpers already present in the alpine package.

## Tests

Mirror the deb github tests: scan derives `.PKGINFO`/Q1 from a ranged prefix of a `testsupport.MinimalApk` served over an httptest range server (no full download); diff/prune; pattern filter; per-arch `ServeRemote` routing (index served per-arch and grouped, dot-segment collapse, `.apk` → 302, cold-start 503, warm 200, canceled-request-serves-cache); DB lease prevents a second replica; prime bypasses the recency window. `go build`/`go vet`/`go mod tidy` clean, `make test` (-race) green, pre-commit green.

Reviewed-on: #115
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

pkg/models/package_type.go

@@ -5,35 +5,37 @@ import "fmt"
 type PackageType string
 
 const (
-	PackageGeneric   PackageType = "generic"
-	PackageDocker    PackageType = "docker"
-	PackageHelm      PackageType = "helm"
-	PackagePyPI      PackageType = "pypi"
-	PackageNPM       PackageType = "npm"
-	PackageRPM       PackageType = "rpm"
-	PackageDeb       PackageType = "deb"
-	PackageAlpine    PackageType = "alpine"
-	PackagePuppet    PackageType = "puppet"
-	PackageTerraform PackageType = "terraform"
-	PackageGoProxy   PackageType = "goproxy"
-	PackageGitHubRPM PackageType = "github_rpm"
-	PackageGitHubDeb PackageType = "github_deb"
+	PackageGeneric      PackageType = "generic"
+	PackageDocker       PackageType = "docker"
+	PackageHelm         PackageType = "helm"
+	PackagePyPI         PackageType = "pypi"
+	PackageNPM          PackageType = "npm"
+	PackageRPM          PackageType = "rpm"
+	PackageDeb          PackageType = "deb"
+	PackageAlpine       PackageType = "alpine"
+	PackagePuppet       PackageType = "puppet"
+	PackageTerraform    PackageType = "terraform"
+	PackageGoProxy      PackageType = "goproxy"
+	PackageGitHubRPM    PackageType = "github_rpm"
+	PackageGitHubDeb    PackageType = "github_deb"
+	PackageGitHubAlpine PackageType = "github_alpine"
 )
 
 var validPackageTypes = map[PackageType]bool{
-	PackageGeneric:   true,
-	PackageDocker:    true,
-	PackageHelm:      true,
-	PackagePyPI:      true,
-	PackageNPM:       true,
-	PackageRPM:       true,
-	PackageDeb:       true,
-	PackageAlpine:    true,
-	PackagePuppet:    true,
-	PackageTerraform: true,
-	PackageGoProxy:   true,
-	PackageGitHubRPM: true,
-	PackageGitHubDeb: true,
+	PackageGeneric:      true,
+	PackageDocker:       true,
+	PackageHelm:         true,
+	PackagePyPI:         true,
+	PackageNPM:          true,
+	PackageRPM:          true,
+	PackageDeb:          true,
+	PackageAlpine:       true,
+	PackagePuppet:       true,
+	PackageTerraform:    true,
+	PackageGoProxy:      true,
+	PackageGitHubRPM:    true,
+	PackageGitHubDeb:    true,
+	PackageGitHubAlpine: true,
 }
 
 func (p PackageType) Valid() bool {

pkg/models/package_type_test.go

@@ -19,6 +19,8 @@ func TestPackageTypeValid(t *testing.T) {
 		models.PackageTerraform,
 		models.PackageGoProxy,
 		models.PackageGitHubRPM,
+		models.PackageGitHubDeb,
+		models.PackageGitHubAlpine,
 	}
 	for _, pt := range valid {
 		if !pt.Valid() {