feat(Evaluation Framework): non-blocking follow-ups from lamb PR #357 (KS + Library Manager)

[NoveliaYuki]

Purpose

Capture the non-blocking follow-ups identified during the production-readiness review of lamb PR #357 (Knowledge Store Server + Library Manager integration). These are safe to merge without addressing, but they should not be lost — each one is a known papercut that will bite a future contributor or operator if left untracked.

The work itself lives in the lamb repository, not in lamb-evals — this issue exists in lamb-evals only because that is where Novelia tracks her LAMB-related work. Any PR that addresses an item here will land in Lamb-Project/lamb, with its own issue + branch over there following that repo's conventions.

Scope is limited to items in the Knowledge Store and Library Manager subsystems. Repo-wide concerns (CI workflow gaps, pre-existing lint debt inherited from main) are explicitly out of scope and tracked elsewhere.

Acceptance criteria

• Library Manager has Alembic migrations wired up (currently uses SQLAlchemy create_all, documented as a known limitation in lamb/CLAUDE.md). Acceptable shape: an alembic/ directory under library-manager/, an initial migration that captures the current schema, and the startup path runs alembic upgrade head instead of Base.metadata.create_all.
• .gitignore entries added so the test artifacts stop showing up as untracked: lamb-cli/.coverage, lamb-kb-server/mutation-results*.txt, test-results/ (root). These appeared during the lifecycle verification and are currently never committed only by accident.
• The lamb-cli --wait exit-code contract change from PR #357 is documented somewhere durable. lamb library upload --wait, lamb library import-url --wait, lamb library import-youtube --wait, and lamb ks add-content --wait now exit non-zero when any item finishes in failed status — previously they exited 0 and only printed to stderr. Either a CHANGELOG entry, an updated CLI help string, or a section in lamb-cli/README.md is acceptable; the goal is that anyone scripting against these commands learns about the change without reading the diff.
• Stale lamb-kb-server/Documentation/issue_334_known_bugs.md already updated in chore(#337): post-merge cleanup (commit 7543a453 on feat/#337/verify-ks-lifecycle). Listed here only for completeness — no further action needed; the box stays unchecked until that PR merges to main.

Author: @NoveliaYuki