fix: date_created default value always set to server start time

[chunlampang]

Description

• date_created default value is always set to server start time

Type of change

• New features (non-breaking change).
• Bug fix (non-breaking change).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).

Checklist

• I have performed a self-review of my code.
• I have added thorough tests if it is a core feature.
• There is a reference to the original bug report and related work.
• I have commented on my code, particularly in hard-to-understand areas.
• The feature is well documented.

[Copilot]

Pull request overview

Fixes incorrect date_created defaults that were being evaluated at import/server start time, ensuring each new row gets the actual creation timestamp at insert time.

Changes:

• Update SQLAlchemy date_created column defaults to use a callable (lambda) so they’re evaluated per-row.
• Apply the fix consistently across both the Chroma migration script schema and the FileIndex dynamic table schemas.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
scripts/migrate/migrate_chroma_db.py	Makes date_created default evaluated at insert time for dynamically created Source tables in the migration script.
libs/ktem/ktem/index/file/index.py	Fixes date_created defaults for Source and FileGroup tables created by FileIndex._setup_resources().

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