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>

frontend/src/stores/fruitStore.ts

@@ -52,6 +52,7 @@ export const useFruitStore = defineStore('fruit', () => {
     const fruit = await createFruit(dto)
     fruits.value = [fruit, ...fruits.value]
     total.value += 1
+    offset.value = 0
     return fruit
   }