feat(schema): add donor_id + database_version catalog columns

[timtreis]

What

Adds two nullable string columns to the shared catalog contract (sddb._catalog_schema.CATALOG_COLUMNS):

• donor_id — a per-dataset donor identifier.
• database_version — the stable schema version an object was curated against (the lamin "Database Version"), so a consumer no longer has to infer a version from the lamin uid suffix.

Why

First of a two-PR sequence. This PR lands the contract so the engine producer (theislab/spatialdata-db) can populate both columns in a follow-up — the producer's inventory_to_catalog can only emit a column that already exists in CATALOG_COLUMNS. The companion engine PR adds the emission (database_version from the Database Version feature; donor_id heuristically from the Replicate feature) and backfills study_id.

Scope / notes

• Both columns are optional (not in REQUIRED_COLUMNS), non-faceted (not in FACETS), non-range (not in RANGE_COLUMNS). They flow through Catalog.to_df() automatically — no reader change.
• Deliberately not wired into freeze.py / compute_fingerprint: database_version is informational, not part of cohort identity. The client keeps its existing internal lamin_version (uid[16:]) unchanged — hence the distinct name database_version rather than lamin_version.

Testing

• New tests/test_catalog_schema.py cases: the two columns are present and typed string, are absent from the required/facet/range sets, validate both populated and all-null, and round-trip through parquet.
• Full suite: 249 passed, 2 skipped; ruff + mypy clean on changed files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.91%. Comparing base (de9cb31) to head (f486619).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   91.67%   91.91%   +0.24%     
==========================================
  Files          19       19              
  Lines        1429     1460      +31     
==========================================
+ Hits         1310     1342      +32     
+ Misses        119      118       -1     

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