feat: deb (Debian/apt) local and remote repository support

Adds a deb provider with feature parity to rpm for local and remote repos,
so .deb packages can be hosted (flat apt repo generated on upload) and a
Debian/Ubuntu mirror can be cached through the proxy.

- add PackageDeb to the package-type enum
- new internal/provider/deb: Classify (.deb immutable, Packages/Release/dists
  mutable), ValidateUpload to pool/<file>, pure-Go .deb parse (ar -> control.tar
  gz/xz/zst -> ./control), and a flat-repo LocalIndexer serving Packages,
  Packages.gz and an unsigned Release ([trusted=yes], mirroring rpm gpgcheck=0)
- remote proxy path: UpstreamURL/ContentType/AuthHeaders via BasicHeaders
- DebMetadata struct + store interfaces on provider; deb_metadata table in
  migrate() and Insert/Delete/List DB methods
- blank-import the deb provider in the server
- testsupport.MinimalDeb pure-Go fixture builder
- unit tests (classify, control parse across gz/xz/zst, Packages/Release
  generation, validate) and a dockere2e TestLocalDebRepo

pkg/models/package_type.go

@@ -11,6 +11,7 @@ const (
 	PackagePyPI      PackageType = "pypi"
 	PackageNPM       PackageType = "npm"
 	PackageRPM       PackageType = "rpm"
+	PackageDeb       PackageType = "deb"
 	PackageAlpine    PackageType = "alpine"
 	PackagePuppet    PackageType = "puppet"
 	PackageTerraform PackageType = "terraform"
@@ -25,6 +26,7 @@ var validPackageTypes = map[PackageType]bool{
 	PackagePyPI:      true,
 	PackageNPM:       true,
 	PackageRPM:       true,
+	PackageDeb:       true,
 	PackageAlpine:    true,
 	PackagePuppet:    true,
 	PackageTerraform: true,