Add Alpine/apk local repository support

The alpine provider hosted only remote (proxy) repos; 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.

- Implement LocalUploader/LocalIndexer/PostUploadHook/PostDeleteHook on the
  alpine provider, keeping the remote proxy methods intact.
- Parse .apk (concatenated gzipped tar streams) in pure Go: read .PKGINFO from
  the control stream and compute the apk pull checksum C: = Q1+base64(sha1) over
  the raw control gzip stream (not the whole file).
- Generate an unsigned per-arch APKINDEX.tar.gz (clients use --allow-untrusted),
  applying the same dot-segment normalization as deb for ./<arch>/... requests.
- Add AlpineMetadata plus separate Alpine store/reader/deleter interfaces so the
  rpm/deb metadata interfaces are not widened.
- Add the alpine_metadata table and its Insert/Delete/List DB methods.
- Add testsupport.MinimalApk plus unit tests (parse, Q1 checksum, per-arch
  filtering, dot-segment handling, validate) and a dockere2e index test.

internal/testsupport/apk.go

@@ -0,0 +1,38 @@
+package testsupport
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// MinimalApk builds a valid-enough Alpine package in pure Go (no committed
+// binary fixture, no abuild): two concatenated, independently gzipped tar
+// streams -- a control stream carrying .PKGINFO and a data stream carrying a
+// single payload file. It mirrors MinimalDeb/MinimalRPM and is parseable by the
+// alpine provider (which derives arch/name/version and the Q1 pull checksum from
+// the control stream).
+func MinimalApk(name, version, arch string) []byte {
+	pkginfo := fmt.Sprintf(
+		"# generated by testsupport\n"+
+			"pkgname = %s\n"+
+			"pkgver = %s\n"+
+			"arch = %s\n"+
+			"pkgdesc = minimal test package\n"+
+			"url = https://example.com/%s\n"+
+			"license = MIT\n"+
+			"origin = %s\n"+
+			"maintainer = e2e <e2e@example.com>\n"+
+			"builddate = 1700000000\n"+
+			"size = 4\n"+
+			"depend = so:libc.musl-x86_64.so.1\n"+
+			"provides = cmd:%s=%s\n",
+		name, version, arch, name, name, name, version)
+
+	control := gzipBytes(tarSingle(".PKGINFO", []byte(pkginfo)))
+	data := gzipBytes(tarSingle("usr/bin/"+name, []byte("body")))
+
+	var buf bytes.Buffer
+	buf.Write(control)
+	buf.Write(data)
+	return buf.Bytes()
+}