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/database/postgres.go

@@ -182,6 +182,35 @@ func (db *DB) migrate() error {
 
 		CREATE INDEX IF NOT EXISTS idx_deb_metadata_repo ON deb_metadata(repo_name);
 
+		CREATE TABLE IF NOT EXISTS alpine_metadata (
+			id                BIGSERIAL PRIMARY KEY,
+			repo_name         TEXT NOT NULL,
+			file_path         TEXT NOT NULL,
+			content_hash      TEXT NOT NULL,
+			checksum          TEXT NOT NULL,
+			name              TEXT NOT NULL,
+			version           TEXT NOT NULL,
+			arch              TEXT NOT NULL,
+			download_size     BIGINT DEFAULT 0,
+			installed_size    BIGINT DEFAULT 0,
+			description       TEXT DEFAULT '',
+			url               TEXT DEFAULT '',
+			license           TEXT DEFAULT '',
+			origin            TEXT DEFAULT '',
+			maintainer        TEXT DEFAULT '',
+			build_time        BIGINT DEFAULT 0,
+			commit_hash       TEXT DEFAULT '',
+			provider_priority TEXT DEFAULT '',
+			depends           TEXT DEFAULT '',
+			provides          TEXT DEFAULT '',
+			install_if        TEXT DEFAULT '',
+			created_at        TIMESTAMPTZ DEFAULT NOW(),
+			UNIQUE(repo_name, file_path)
+		);
+
+		CREATE INDEX IF NOT EXISTS idx_alpine_metadata_repo ON alpine_metadata(repo_name);
+		CREATE INDEX IF NOT EXISTS idx_alpine_metadata_repo_arch ON alpine_metadata(repo_name, arch);
+
 		CREATE TABLE IF NOT EXISTS github_rpm_sync_state (
 			remote_name        TEXT PRIMARY KEY,
 			etag               TEXT DEFAULT '',