fix: correct UUID format string issues in logging and tests

- Fix scheduler.go log.Printf calls to convert pgtype.UUID to string before formatting
- Fix ebook.go fmt.Printf calls to convert pgtype.UUID to string before formatting
- Add missing Enabled field to rate limiter config in security test
- Prevents format string errors when logging library IDs

This resolves compilation errors where pgtype.UUID was being formatted
with %s which expects a string, not a UUID struct.

cmd/server/tests/security_test.go

@@ -87,6 +87,7 @@ func TestRateLimiterSecurity(t *testing.T) {
 	e := echo.New()
 
 	config := ratelimit.RateLimiterConfig{
+		Enabled:           true,
 		RequestsPerMinute: 3,
 		CleanupInterval:   1 * time.Minute,
 	}