deb/apk: make local repodata deterministic

Part of #117. The two no-affinity replicas (and every regeneration) must
serve byte-identical local repodata so apt/apk never hit a checksum
mismatch between an index's advertised hash and the bytes actually served.

- Derive the deb Release Date: from the newest persisted created_at
  (RFC1123Z, UTC) instead of time.Now(); carry created_at through the deb
  metadata SELECT and DebMetadata struct.
- Pin the apk APKINDEX tar header ModTime to the Unix epoch instead of the
  zero-value time.Time, so it is never wall-clock derived.
- Give both list queries a genuine total order by adding a file_path
  tiebreak (name/version/arch is not unique).
- Add guard tests: deb generators byte-identical across generations, the
  Release checksum/size invariant matches the served Packages(.gz) bytes,
  the Date: is pinned to created_at; apk index byte-identical and tar
  ModTime pinned to epoch.

internal/database/alpine_metadata.go

@@ -41,7 +41,7 @@ func (db *DB) ListAlpineMetadataEntries(ctx context.Context, repoName string) ([
 			depends, provides, install_if
 		FROM alpine_metadata
 		WHERE repo_name = $1
-		ORDER BY name, version, arch
+		ORDER BY name, version, arch, file_path
 	`, repoName)
 	if err != nil {
 		return nil, err