feat(US5+6): QA generation — POST /api/v1/qa/gen-text and /gen-image

- Add qa_models.py with TextQAItem, GenTextQARequest, QAPair, ImageQAItem,
  GenImageQARequest, ImageQAPair, TextQAResponse, ImageQAResponse
- Implement gen_text_qa(): batch-formats triples into a single prompt, calls
  llm.chat(), parses JSON array via extract_json()
- Implement gen_image_qa(): downloads cropped image from source-data bucket,
  base64-encodes inline (data URI), builds multimodal message, calls
  llm.chat_vision(), parses JSON; image_path preserved on ImageQAPair
- Replace qa.py stub with full router: POST /qa/gen-text and /qa/gen-image
  using Depends(get_llm_client) and Depends(get_storage_client)
- 15 new tests (8 service + 7 router), 53/53 total passing

tests/test_qa_router.py

@@ -0,0 +1,121 @@
+"""Tests for QA router: /api/v1/qa/gen-text and /api/v1/qa/gen-image."""
+import json
+import pytest
+from unittest.mock import AsyncMock
+
+from app.core.exceptions import LLMCallError, LLMParseError, StorageError
+
+
+SAMPLE_QA_JSON = json.dumps([
+    {"question": "电缆接头位于哪里？", "answer": "配电箱左侧"},
+])
+
+FAKE_IMAGE_BYTES = b"\xff\xd8\xff\xe0fake_jpeg_content"
+
+TEXT_QA_PAYLOAD = {
+    "items": [
+        {
+            "subject": "电缆接头",
+            "predicate": "位于",
+            "object": "配电箱左侧",
+            "source_snippet": "电缆接头位于配电箱左侧",
+        }
+    ]
+}
+
+IMAGE_QA_PAYLOAD = {
+    "items": [
+        {
+            "subject": "电缆接头",
+            "predicate": "位于",
+            "object": "配电箱左侧",
+            "cropped_image_path": "crops/1/0.jpg",
+        }
+    ]
+}
+
+
+# ---------------------------------------------------------------------------
+# POST /api/v1/qa/gen-text
+# ---------------------------------------------------------------------------
+
+
+def test_gen_text_qa_returns_200(client, mock_llm):
+    mock_llm.chat = AsyncMock(return_value=SAMPLE_QA_JSON)
+
+    resp = client.post("/api/v1/qa/gen-text", json=TEXT_QA_PAYLOAD)
+
+    assert resp.status_code == 200
+    data = resp.json()
+    assert "pairs" in data
+    assert len(data["pairs"]) == 1
+    assert data["pairs"][0]["question"] == "电缆接头位于哪里？"
+    assert data["pairs"][0]["answer"] == "配电箱左侧"
+
+
+def test_gen_text_qa_llm_parse_error_returns_502(client, mock_llm):
+    mock_llm.chat = AsyncMock(return_value="not valid json {{")
+
+    resp = client.post("/api/v1/qa/gen-text", json=TEXT_QA_PAYLOAD)
+
+    assert resp.status_code == 502
+    assert resp.json()["code"] == "LLM_PARSE_ERROR"
+
+
+def test_gen_text_qa_llm_call_error_returns_503(client, mock_llm):
+    mock_llm.chat = AsyncMock(side_effect=LLMCallError("GLM timeout"))
+
+    resp = client.post("/api/v1/qa/gen-text", json=TEXT_QA_PAYLOAD)
+
+    assert resp.status_code == 503
+    assert resp.json()["code"] == "LLM_CALL_ERROR"
+
+
+# ---------------------------------------------------------------------------
+# POST /api/v1/qa/gen-image
+# ---------------------------------------------------------------------------
+
+
+def test_gen_image_qa_returns_200(client, mock_llm, mock_storage):
+    mock_storage.download_bytes = AsyncMock(return_value=FAKE_IMAGE_BYTES)
+    mock_llm.chat_vision = AsyncMock(return_value=SAMPLE_QA_JSON)
+
+    resp = client.post("/api/v1/qa/gen-image", json=IMAGE_QA_PAYLOAD)
+
+    assert resp.status_code == 200
+    data = resp.json()
+    assert "pairs" in data
+    assert len(data["pairs"]) == 1
+    pair = data["pairs"][0]
+    assert pair["question"] == "电缆接头位于哪里？"
+    assert pair["answer"] == "配电箱左侧"
+    assert pair["image_path"] == "crops/1/0.jpg"
+
+
+def test_gen_image_qa_llm_parse_error_returns_502(client, mock_llm, mock_storage):
+    mock_storage.download_bytes = AsyncMock(return_value=FAKE_IMAGE_BYTES)
+    mock_llm.chat_vision = AsyncMock(return_value="bad json {{")
+
+    resp = client.post("/api/v1/qa/gen-image", json=IMAGE_QA_PAYLOAD)
+
+    assert resp.status_code == 502
+    assert resp.json()["code"] == "LLM_PARSE_ERROR"
+
+
+def test_gen_image_qa_llm_call_error_returns_503(client, mock_llm, mock_storage):
+    mock_storage.download_bytes = AsyncMock(return_value=FAKE_IMAGE_BYTES)
+    mock_llm.chat_vision = AsyncMock(side_effect=LLMCallError("GLM vision timeout"))
+
+    resp = client.post("/api/v1/qa/gen-image", json=IMAGE_QA_PAYLOAD)
+
+    assert resp.status_code == 503
+    assert resp.json()["code"] == "LLM_CALL_ERROR"
+
+
+def test_gen_image_qa_storage_error_returns_502(client, mock_storage):
+    mock_storage.download_bytes = AsyncMock(side_effect=StorageError("RustFS down"))
+
+    resp = client.post("/api/v1/qa/gen-image", json=IMAGE_QA_PAYLOAD)
+
+    assert resp.status_code == 502
+    assert resp.json()["code"] == "STORAGE_ERROR"