Fix integration tests leaving original source propositions behind

Add cleanup for original_source_proposition_id from both
proposition_original_source_links and document_original_source_links
tables. These system-created propositions (creator_id = 0) were not
being collected for deletion, causing them to remain as orphans after
test cleanup.

Closes #127

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>