Add Alpine/apk local repository support (#114)

## Why

The alpine provider only supported remote (proxy) repositories, so there was no way to publish first-party `.apk` packages the way `rpm-local` and `deb-local` already allow. This extends the existing alpine provider into a real apk repository: uploaded `.apk` files are parsed in pure Go and a per-arch `APKINDEX.tar.gz` is generated on demand, at parity with rpm repodata and deb Packages generation. (The metadata-only `github_alpine` type is a separate follow-up and is not part of this PR.)

## How

- Implements `LocalUploader` / `LocalIndexer` / `PostUploadHook` / `PostDeleteHook` on the existing `alpine` provider, leaving the remote proxy methods (`UpstreamURL`/`ContentType`/`AuthHeaders`/`RewriteResponse`/`Classify`) intact.
- Parses the `.apk` (up to three concatenated, independently gzipped tar streams) in pure Go: locates the control stream by its `.PKGINFO` member, reads the `key = value` fields, and computes the apk pull checksum `C:` = `Q1` + base64(sha1(**control gzip stream bytes**)) — the sha1 of the second gzip member, not of the whole file.
- Derives arch from `.PKGINFO` and records download size (`S:` blob size) and installed size (`I:` from `.PKGINFO size`).
- Generates an **unsigned** per-arch `APKINDEX.tar.gz` = gzip(tar(`APKINDEX`)) filtered by requested arch (clients use `--allow-untrusted`, matching rpm `gpgcheck=0` / deb `[trusted=yes]`), applying the same dot-segment normalization as deb so `./<arch>/APKINDEX.tar.gz` resolves. Non-index / `.apk` paths return `false` so the generic file streamer serves the stored blob.
- Adds `AlpineMetadata` plus **separate** `AlpineMetadataStore` / `AlpineMetadataReader` / `AlpineMetadataDeleter` interfaces (type-asserted from the generic hooks) so the shared rpm/deb metadata interfaces and their test doubles are untouched.
- Adds the `alpine_metadata` table (keyed by `repo_name` + `file_path`, per-arch index) and its `Insert`/`Delete`/`List` DB methods.
- Adds `testsupport.MinimalApk`, unit tests (`.PKGINFO` parse, Q1 checksum over the control stream, per-arch filtering, empty-field omission, `./` dot-segment handling, ValidateUpload accept/reject), and a `dockere2e` `TestLocalAlpineIndex`.

## Consumption

`/etc/apk/repositories` line = `<url>/api/v1/local/<name>` (apk appends `/<arch>/APKINDEX.tar.gz`); `apk update --allow-untrusted && apk add --allow-untrusted <pkg>`. Packages live at `/api/v1/local/<name>/<arch>/<file>.apk`.

## Verification

`go build ./...`, `go vet ./...` (incl. `-tags dockere2e`), `go mod tidy` (no change), `make test` (`-race`), and `pre-commit run --all-files` all pass.

Reviewed-on: #114
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

e2e-docker/local_test.go

@@ -3,7 +3,10 @@
 package e2edocker
 
 import (
+	"archive/tar"
 	"bytes"
+	"compress/gzip"
+	"io"
 	"net/http"
 	"strings"
 	"testing"
@@ -136,3 +139,55 @@ func TestLocalDebRepo(t *testing.T) {
 		t.Fatalf("deb content mismatch")
 	}
 }
+
+// TestLocalAlpineIndex uploads an .apk to an alpine local repo and validates
+// that a per-arch APKINDEX.tar.gz is generated automatically from the parsed
+// .PKGINFO (the apk-local analog of rpm repodata / deb Packages generation).
+func TestLocalAlpineIndex(t *testing.T) {
+	createRepo(t, `{"name":"local-alpine","package_type":"alpine","repo_type":"local"}`)
+	defer deleteRepo(t, "local-alpine")
+
+	apk := testsupport.MinimalApk("e2e-testpkg", "1.0-r0", "x86_64")
+	uploadFile(t, "local-alpine", "x86_64/e2e-testpkg-1.0-r0.apk", apk, "application/vnd.android.package-archive")
+
+	// The index is generated asynchronously after upload; poll for it.
+	resp, body := getEventually(t, api("/api/v1/local/local-alpine/x86_64/APKINDEX.tar.gz"), 15*time.Second)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("APKINDEX: status %d: %s", resp.StatusCode, body)
+	}
+
+	zr, err := gzip.NewReader(bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("APKINDEX not gzip: %v", err)
+	}
+	tarBytes, _ := io.ReadAll(zr)
+	tr := tar.NewReader(bytes.NewReader(tarBytes))
+	var index string
+	for {
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("APKINDEX not tar: %v", err)
+		}
+		if hdr.Name == "APKINDEX" {
+			b, _ := io.ReadAll(tr)
+			index = string(b)
+		}
+	}
+	for _, want := range []string{"P:e2e-testpkg", "V:1.0-r0", "A:x86_64", "C:Q1", "S:", "I:"} {
+		if !strings.Contains(index, want) {
+			t.Fatalf("APKINDEX missing %q:\n%s", want, index)
+		}
+	}
+
+	// The .apk downloads back byte-identical from its arch path.
+	resp, body = doRequest(t, http.MethodGet, api("/api/v1/local/local-alpine/x86_64/e2e-testpkg-1.0-r0.apk"), nil, "")
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("download apk: status %d: %s", resp.StatusCode, body)
+	}
+	if !bytes.Equal(body, apk) {
+		t.Fatalf("apk content mismatch")
+	}
+}