Fix code review findings: image read, whitespace validation, offset reset, cleanup error

- Replace src.Read(data) with io.ReadAll(src) to prevent silent truncation on large uploads
- Reject zero-byte file uploads with 400 rather than storing empty BYTEA
- Add strings.TrimSpace to validateFruit so whitespace-only name/osdb_number returns 422
- Reset offset to 0 in fruitStore.create() so navigating back to list after create shows page 1
- Log t.Cleanup DELETE error in integration test to surface constraint failures clearly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/repository/fruit_repo_integration_test.go

@@ -41,7 +41,9 @@ func TestFruitRepoIntegration(t *testing.T) {
 
 	// cleanup after test
 	t.Cleanup(func() {
-		pool.Exec(ctx, `DELETE FROM fruits WHERE osdb_number LIKE 'TEST-%'`)
+		if _, err := pool.Exec(ctx, `DELETE FROM fruits WHERE osdb_number LIKE 'TEST-%'`); err != nil {
+			t.Logf("cleanup: %v", err)
+		}
 	})
 
 	// Create