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.

internal/database/queries/queries.sql

@@ -373,7 +373,7 @@ LEFT JOIN library_visibility lv ON l.id = lv.library_id AND lv.user_id = sqlc.na
 LIMIT sqlc.narg('limit') OFFSET sqlc.narg('offset');
 
 -- name: SearchMediaItemsUnified :many
-SELECT DISTINCT ON (mi.id)
+SELECT
   mi.id,
   mi.library_id,
   mi.title,
@@ -495,7 +495,6 @@ WHERE COALESCE(lv.is_visible, true) = true
     )
   )
 ORDER BY
-  mi.id,
   -- Primary sort: relevance score when searching
   CASE
     WHEN sqlc.narg('search_query') != '' THEN