fix: implement proper 3-state boolean logic for has_cover filter

Fixed the SearchMediaItemsUnified query to properly handle the has_cover
parameter in three states:
- NULL (not specified): Show all books
- TRUE: Show only books with cover images
- FALSE: Show only books without cover images

Changes:
- Added explicit boolean casting (::bool) to sqlc.narg('has_cover')
  to resolve PostgreSQL type inference error (SQLSTATE 42P08)
- Replaced single AND condition with OR'd logic to handle all three
  states without mutual exclusion
- Used IS NULL check to detect when parameter is not specified
- Used IS TRUE/IS FALSE to explicitly check boolean states

The previous implementation had mutually exclusive AND conditions that
prevented any records from matching when has_cover was explicitly set
to TRUE or FALSE, causing the filter to block all searches.

This fix resolves the issue where searches were returning 0 results
regardless of other filter parameters when has_cover was included in
the query.

internal/database/queries.sql.go

@@ -7271,7 +7271,11 @@ WHERE COALESCE(lv.is_visible, true) = true
     (mi.date_published IS NULL AND mi.copyright_year <= $9)
   )
   -- Boolean (exact)
-  AND ($10 = false OR mi.cover_image_path IS NOT NULL)
+  AND (
+    $10::bool IS NULL OR  -- Not specified = show all
+    ($10::bool IS TRUE AND mi.cover_image_path IS NOT NULL) OR
+    ($10::bool IS FALSE AND mi.cover_image_path IS NULL)
+  )
   -- Search query (fuzzy or exact based on quotes)
   AND (
     $11 = '' OR
@@ -7376,7 +7380,7 @@ type SearchMediaItemsUnifiedParams struct {
 	TagsFilter     interface{} `db:"tags_filter" json:"tags_filter"`
 	YearMin        interface{} `db:"year_min" json:"year_min"`
 	YearMax        interface{} `db:"year_max" json:"year_max"`
-	HasCover       interface{} `db:"has_cover" json:"has_cover"`
+	HasCover       pgtype.Bool `db:"has_cover" json:"has_cover"`
 	SearchQuery    interface{} `db:"search_query" json:"search_query"`
 	IsExactSearch  interface{} `db:"is_exact_search" json:"is_exact_search"`
 	Sort           interface{} `db:"sort" json:"sort"`