feat(knowledge): 分类可改名/配色/删除，选中态与加载态更清楚

分类之前只能新增，建错了只能干看着。补上三件事：

- knowledge_category 加 color 列（迁移 0009，CHECK 卡 #RRGGBB），
  PATCH 支持改名与改色，空串表示取消上色；知识列表带出分类色，前端不用再查。
- 分类行悬停出「⋯」：改名、选色板/手填色值、建子分类、删除（有知识时确认后 force）。
- 选中分类改成主色浅底 + 描边 + 色点，列表头显示当前筛选；切分类时列表淡出并转圈，
  不再一动不动等请求回来。

顺带给卡片画分类色上边线：扫一眼就能分出哪几条属于同一类。

backend/tests/test_knowledge_categories.py

@@ -329,3 +329,86 @@ async def test_categories_come_back_as_a_tree(client: AsyncClient) -> None:
         "技术开发 / 技术 / 部署",
         "餐饮业务",
     ]
+
+
+@pytest.mark.asyncio
+async def test_category_color_roundtrip(client: AsyncClient) -> None:
+    """分类色：建时能给、能改、能取消，非法值当场拒绝。
+
+    颜色是直接拼进 CSS 的（卡片上边线），所以格式必须在入口挡住：
+    一个 `red` 或 `#4f46e5cc` 进了库，前端只会静默退化成灰边框，没人知道为什么。
+    """
+    user, ws_id = await _ws(client)
+    base = f"{V1}/workspaces/{ws_id}/knowledge/categories"
+
+    created = unwrap(
+        await client.post(json={"name": "部署", "color": "#4f46e5"}, url=base, headers=user["headers"])
+    )
+    # 统一成大写，前端不用再 casefold
+    assert created["color"] == "#4F46E5"
+
+    listed = unwrap(await client.get(base, headers=user["headers"]))["items"]
+    assert [c["color"] for c in listed] == ["#4F46E5"]
+
+    recolored = unwrap(
+        await client.patch(
+            f"{base}/{created['id']}",
+            json={"color": "#0EA5E9", "version": created["version"]},
+            headers=user["headers"],
+        )
+    )
+    assert recolored["color"] == "#0EA5E9"
+    assert recolored["name"] == "部署"  # 只改色不动名字
+
+    cleared = unwrap(
+        await client.patch(
+            f"{base}/{created['id']}",
+            json={"color": "", "version": recolored["version"]},
+            headers=user["headers"],
+        )
+    )
+    assert cleared["color"] is None
+
+    # 不传 color 的普通改名不该顺手把颜色清掉
+    colored = unwrap(
+        await client.patch(
+            f"{base}/{created['id']}",
+            json={"color": "#10B981", "version": cleared["version"]},
+            headers=user["headers"],
+        )
+    )
+    renamed = unwrap(
+        await client.patch(
+            f"{base}/{created['id']}",
+            json={"name": "上线", "version": colored["version"]},
+            headers=user["headers"],
+        )
+    )
+    assert renamed["name"] == "上线"
+    assert renamed["color"] == "#10B981"
+
+    bad = await client.post(json={"name": "业务", "color": "red"}, url=base, headers=user["headers"])
+    assert bad.status_code == 400, bad.text
+    assert _code(bad) == 1002
+
+    too_long = await client.post(
+        json={"name": "业务2", "color": "#4f46e5cc"}, url=base, headers=user["headers"]
+    )
+    assert too_long.status_code == 400, too_long.text
+
+
+@pytest.mark.asyncio
+async def test_knowledge_card_carries_category_color(client: AsyncClient) -> None:
+    """知识列表里带上分类（含颜色），前端画卡片上边线不用再查一次。"""
+    user, ws_id = await _ws(client)
+    cat = unwrap(
+        await client.post(
+            f"{V1}/workspaces/{ws_id}/knowledge/categories",
+            json={"name": "复盘", "color": "#EF4444"},
+            headers=user["headers"],
+        )
+    )
+    await _note(client, user, ws_id, "一次事故复盘", category_id=cat["id"])
+
+    page = unwrap(await client.get(f"{V1}/workspaces/{ws_id}/knowledge", headers=user["headers"]))
+    assert page["items"][0]["category"]["color"] == "#EF4444"