Add github_alpine metadata-only package type

github_alpine is the Alpine/apk analog of github_deb/github_rpm: a
metadata-only remote that scans a GitHub repo's releases for .apk assets,
derives each package's .PKGINFO via a ranged prefix fetch (never
downloading whole packages), synthesizes a per-arch APKINDEX.tar.gz from
that cached metadata, and 302-redirects .apk downloads to a backend
releases_remote. It stacks on the apk-local work, reusing the alpine
provider's APKINDEX generator, .PKGINFO parser, Q1 checksum, and
AlpineMetadata store.

- pkg/models: add PackageGitHubAlpine to the enum + validators
- internal/provider/alpine/github.go: the github_alpine provider
  (ServeRemote per-arch index + .apk redirect, cold-start 503,
  scanWithState incremental derive, ranged .PKGINFO prefix fetch with
  range-doubling on truncation)
- internal/provider/alpine/syncer.go: parallel background Syncer
  (worker pool, shared limiter, deduped queue, DB lease)
- internal/database/alpine_github_sync.go + github_alpine_sync_state
  table: remote enumeration + per-remote sync lease
- internal/api/v2/remotes.go: primed on create via the shared Primer map
- internal/server/server.go: construct + Run the alpine syncer, register
  it in the Primer map
- tests mirror the deb github_test/syncer_test (scan/diff/prune, ranged
  .PKGINFO parse, per-arch ServeRemote routing, .apk 302, DB lease)

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() {