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>

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 '',