Add study abstract/metadata collection columns to the catalog schema

[timtreis]

What

Adds seven optional, nullable string columns to COLLECTION_COLUMNS in sddb._catalog_schema so collection records can carry a study's abstract + bibliographic metadata:

abstract, abstract_license, abstract_source, study_title, study_authors, study_year, study_journal

Additive only — COLLECTION_REQUIRED is unchanged, existing columns untouched. tests/_fixtures.py padded to the new column count (assertion-neutral).

Part of a 3-repo arc — merge this first

This is the shared contract the other two depend on:

• engine spatialdata-db#<engine PR> fills these columns (EPMC/bioRxiv fetch + license gate). Its catalog-producer test only passes once this is released/installed.
• web spatialdata-db-web#28 projects + renders them.

After this merges and a new sddb is released, the engine CI goes green and sddb publish-catalog starts emitting the fields.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.91%. Comparing base (a6e2151) to head (bfd66c5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #18   +/-   ##
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  Coverage   91.91%   91.91%           
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  Files          19       19           
  Lines        1460     1460           
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  Hits         1342     1342           
  Misses        118      118           

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