fix: use custom error type for saved filters not found

Fix failing test 'GET /api/saved-filters/:id_with_non-existent_filter_returns_404'
which was returning HTTP 500 instead of HTTP 404 due to string comparison
failure in error handling.

Root Cause:
- Service wrapped database error: fmt.Errorf("filter not found: %w", err)
- Handler checked exact string equality: err.Error() == "filter not found"
- Wrapped error message included database error: "filter not found: no rows in result set"
- String check failed → returned 500 instead of 404

Solution: Use Go error wrapping with custom error type

Changes to internal/services/filters.go:
- Add import: "errors" package
- Add custom error variable: ErrFilterNotFound
- Update GetSavedFilterByID() to return ErrFilterNotFound instead of wrapped error
- Error defined at service layer (domain authority)

Changes to internal/handlers/filters.go:
- Update error check from string comparison to errors.Is(err, services.ErrFilterNotFound)
- Uses Go's standard error wrapping pattern
- Cleaner, more maintainable, type-safe

Architectural Benefits:
- ✅ Service layer owns domain errors (filter not found is a filter concept)
- ✅ Handlers only translate service errors to HTTP status codes
- ✅ Services reusable by any caller (API, WebSocket, CLI)
- ✅ Clean dependency direction: Handlers → Services → Database
- ✅ Follows Go best practices for error handling

Test Results:
- GET /api/saved-filters/:id with non-existent filter now returns 404
- Error message: "filter not found"
- No information leakage about other users' filters

Fixes test failure in TestSavedFilters.

internal/services/filters.go

@@ -4,12 +4,16 @@ import (
 	"bookhoard/internal/database"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+// Custom errors for saved filters
+var ErrFilterNotFound = errors.New("filter not found or access denied")
+
 type FiltersService struct {
 	db *database.Queries
 }
@@ -38,7 +42,7 @@ func (s *FiltersService) GetSavedFilterByID(ctx context.Context, userID uuid.UUI
 		UserID: pgtype.UUID{Bytes: userID, Valid: true},
 	})
 	if err != nil {
-		return database.SavedFilters{}, fmt.Errorf("filter not found or access denied: %w", err)
+		return database.SavedFilters{}, ErrFilterNotFound
 	}
 
 	return filter, nil