Restructure ORM: normalize Manifest / ManifestEntry / Archive

[iparask]

Replaces the single ArchiveQueue table (which pickled the whole manifest into a PickleType column) with three real tables — Manifest, ManifestEntry, and Archive (renamed from ArchiveQueue) — wired by real foreign keys with cascade deletes and a 1:1
Manifest↔Archive link. The in-memory DTO formerly named Archive is renamed ArchiveJob to free up the name. Schema style follows the sibling Librarian codebase (db. namespace, column docstrings, locked dequeue via with_for_update(skip_locked=True) filtering
both consumed and completed).

The consumers are cut over to the new shape: the /archive route builds a Manifest via get_or_create plus ManifestEntry rows and one Archive job in a single commit, and the worker rebuilds the manifest dict from item.manifest.entries (eager-loaded) instead of unpickling a blob, snapshots archive_root/archive_path per row, and takes a session_maker parameter while returning bool — two seams that let a future task-based runner drop in. Alembic is introduced with a single baseline revision stamping the three tables;

create_all() is retained for the test suite.

🤖 Assisted by Claude Sonnet 5 and Opus 4.8