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/router/frontend.go

@@ -212,7 +212,7 @@ func registerFrontendRoutes(cfg *Config) {
 					LanguageFilter: "",
 					YearMin:        0,
 					YearMax:        0,
-					HasCover:       false,
+					HasCover:       pgtype.Bool{Valid: false},
 					Sort:           "created_at DESC",
 					Limit:          limit,
 					Offset:         offset,