Add multiple named feeds per user feature

Implements Issue #134: Allow users to have multiple named feeds, each with
its own combination of feed generators and follow lists.

Database changes:
- Add user_feeds table to store named feed configurations
- Add feed_id to user_feed_generators, user_active_follow_lists, feed_view_positions
- Create auto-trigger to create default "My Feed" for new users
- Backfill existing users with default feeds
- Update unique constraints to scope by feed_id

API changes:
- Add get_user_feeds, get_feed, create_feed, update_feed, delete_feed, duplicate_feed
- Update get_user_feed_generators to accept optional feed_id
- Update add_generator_to_feed to accept optional feed_id
- Update feed view tracking functions to accept optional feed_id

UI changes:
- Add PvFeedSelector component for feed selection and management
- Update PvUserFeedViewer to include feed selector
- Update PvFeedEventList, PvFeedNewCount, PvFeedManagement to accept feedId prop
- Add UserFeed and UserFeedResult types to API client

Closes #134

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