fix: generate queryable indexes on the split properties column.

[pantierra]

Technical Context

• Breaking Changes: No

Description

items has no content column; properties live in the split properties jsonb column. indexdef() is still emitting:

to_text(((content -> 'properties') -> 'foo'))

That would fail (or at best be useless) when building queryable indexes. Pointing at properties -> ... to fix.

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Checklist

• Linting: Does not apply for SQL-only change.
• Tests: Tests pass. I have included new tests for these changes where applicable.
• Edge Cases: I have manually verified "unhappy paths" and edge cases beyond the basic success criteria (e.g., database connection timeouts, malformed input, strict mapping rejections).
• Documentation: Added to CHANGELOG.md. No need for more documentation.
• Accountability: I can explain the implementation logic for every line of code submitted.

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AI tool usage

• AI (Copilot or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated.

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Policy: We require a "human-in-the-loop." You are the author and are fully accountable for all submitted code. Please ensure all tool-generated content is thoroughly reviewed before submission to ensure it is not an "extractive contribution" that squanders maintainer time.