add composite index on events table

[TanishqPorwar]

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Link to Issue or Description of Change

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-Related: #4827

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Problem:
Events table lacks an index for the primary query pattern WHERE app_name=? AND user_id=? AND session_id=? ORDER BY timestamp DESC, causing full table scans on PostgreSQL

Solution:
Add a SQLAlchemy Index to the v1 schema model, which gets applied by create_all() on new databases

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