fix: implement proper 3-state boolean handling in backend

Updated the backend services and handlers to properly detect and pass
the has_cover parameter's validity state to the database layer.

Changes:
- services/search.go: Changed HasCover type from bool to pgtype.Bool
  to support 3-state logic (NULL, TRUE, FALSE)
- handlers/media.go: Fixed 3-state detection by checking if has_cover
  exists in query params before setting Valid flag
- router/search.go: Fixed 3-state detection to match media.go logic
- router/frontend.go: Use pgtype.Bool{Valid: false} for SSR initial
  load to ensure no filtering occurs on first page load

The key fix is detecting whether the has_cover parameter was actually
sent in the request:
- Parameter not sent → pgtype.Bool{Bool: false, Valid: false}
- Parameter sent as "true" → pgtype.Bool{Bool: true, Valid: true}
- Parameter sent as "false" → pgtype.Bool{Bool: false, Valid: true}

Previously, media.go was hardcoding Valid: true, which meant it was
always filtering by has_cover=false (only books without covers) when
the parameter wasn't sent, causing searches to incorrectly return
0 results for queries like "1984".

This ensures consistency between the JSON API endpoint (media.go) and
the HTML endpoint (search.go), and fixes the critical bug where SSR
was returning 0 books on initial page load.

internal/services/search.go

@@ -30,7 +30,7 @@ type SearchParams struct {
 	LanguageFilter string
 	YearMin        int
 	YearMax        int
-	HasCover       bool
+	HasCover       pgtype.Bool
 	SearchQuery    string
 	Sort           string
 	Limit          int
@@ -76,7 +76,7 @@ func (s *SearchService) SearchMediaItemsUnified(ctx context.Context, params Sear
 		LanguageFilter: pgtype.Text{String: params.LanguageFilter, Valid: true},
 		YearMin:        pgtype.Int4{Int32: int32(params.YearMin), Valid: true},
 		YearMax:        pgtype.Int4{Int32: int32(params.YearMax), Valid: true},
-		HasCover:       pgtype.Bool{Bool: params.HasCover, Valid: true},
+		HasCover:       params.HasCover,
 		SearchQuery:    pgtype.Text{String: searchQuery, Valid: true},
 		IsExactSearch:  pgtype.Bool{Bool: isExact, Valid: true},
 		// SearchPattern:  pgtype.Text{String: searchPattern, Valid: isExact},