Add database migrations for public feed, metrics, and deduplication

- Add is_public column to episodes table - Add user_episodes
junction table for many-to-many relationship - Add episode_metrics
table for tracking engagement - Add original_url_hash column for
deduplication - Add Core.py functions for public episodes (mark_public,
get_public_episodes) - Add Core.py functions for user_episodes
(add_episode_to_user, user_has_episode, get_user_episodes) -
Add Core.py functions for metrics tracking (track_episode_event,
get_episode_metrics) - Add URL normalization and hashing utilities -
All tests passing

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