fix: add unique constraint for saved filter names and fix search sort ordering

Database changes:
- Add unique index on saved_filters(user_id, name, resource_type)
  Prevents duplicate filter names while allowing same name across
  different users or different resource types

Search functionality fix:
- Remove DISTINCT ON (mi.id) from SearchMediaItemsUnified query
- Remove mi.id from ORDER BY clause (was required by DISTINCT ON)
- This allows user-selected sort field to be primary sort criteria
- Previously results were always sorted by ID first, making sort
  dropdown ineffective
- Relevance score and title remain as fallback sorts

This fixes the sort dropdown functionality on the bookshelf page
where changing the sort option appeared to have no effect.

database/schema/schema.sql

@@ -1025,6 +1025,13 @@ CREATE INDEX IF NOT EXISTS idx_saved_filters_user_resource ON saved_filters(user
 -- Index for name searches (future feature)
 CREATE INDEX IF NOT EXISTS idx_saved_filters_name ON saved_filters(user_id, name);
 
+-- Unique constraint: One filter name per user per resource type
+-- Prevents duplicate filter names while allowing:
+-- - Same name for different users
+-- - Same name for different resources (media-items vs collections)
+CREATE UNIQUE INDEX IF NOT EXISTS idx_saved_filters_user_name_resource 
+ON saved_filters(user_id, name, resource_type);
+
 -- Trigger to auto-update updated_at timestamp
 CREATE OR REPLACE FUNCTION update_updated_at_column()
 RETURNS TRIGGER AS $$