Optimize search functions to filter on important fields

Summary

Optimize all search functions to filter on important fields. Currently, searching for posts by a user returns no results when it should. Do not implement full-text search or embeddings; keep it simple with existing infrastructure.

Steps to Reproduce

1. Navigate to the search feature.
2. Search for posts by a specific user.
3. Observe that no results are returned, even though matching posts exist.

Expected Behaviour

Search functions should correctly filter on important fields (e.g., user, content) and return relevant results.

Actual Behaviour

Search returns no results for queries that should match existing data.

Environment

Not provided.

Reported by retoor via DevPlace.