Add report support for Posts and Comments and fix report creation bugs

[google-labs-jules]

This PR addresses the issue of missing report support for posts and comments by fixing bugs in how reports are created and retrieved.

Changes:

1. Refactored Report retrieval methods: Updated get_pending_reports and get_recent_reports in backend/models/report.py to filter by topic_id correctly, with a fallback to legacy reported_message_id lookup to ensure existing reports are still visible. Updated get_user_reports to check reported_user_id.
2. Fixed Report Creation: Corrected the create_report calls in backend/models/post.py, backend/models/comment.py, and backend/models/message.py to use keyword arguments, ensuring that content_type, content_id, topic_id, and reported_user_id are passed correctly. Previously, incorrect positional arguments were leading to corrupted report data.
3. Clarified Enrichment Logic: Added comments to _enrich_report_with_details in backend/models/report.py to explicitly document the logic for fetching post and comment details.
4. Verified: Added a reproduction/test script backend/repro_report.py (which was run and then deleted) to verify the fixes.

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PR created automatically by Jules for task 15231206498711219601 started by @8JP8

[google-labs-jules]

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