feat: 出厂默认与快照深合并（E4 配置演进）

- app/platform/defaults.py：DEFAULT_CONFIG（spec §5.1 九组，brand/wall 待 Task 4 填充）+ deep_merge（补缺/不动入参/兄弟保留）
- app/platform/tenant_config.py：TenantConfig/Site frozen dataclass；site.brand 以租户级 data["brand"] 为合并基线，租户自定义可被站点继承
- tests/test_config_merge.py：merge 三性质 + 站点覆盖/继承/无 sites 共 5 用例

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

tests/test_config_merge.py

@@ -0,0 +1,68 @@
+"""出厂默认与快照深合并（E4 配置演进）——Task 3 TDD 用例。
+
+覆盖：
+- deep_merge 三性质：缺省补齐 / 不动入参 / 兄弟键保留
+- TenantConfig.from_snapshot 的站点模型：覆盖生效、未覆盖项继承租户级 brand、
+  无 sites 时空元组
+"""
+from app.platform.defaults import DEFAULT_CONFIG, deep_merge
+from app.platform.tenant_config import Site, TenantConfig
+
+
+# ---------- deep_merge 三性质 ----------
+
+def test_missing_keys_filled_from_defaults():
+    snapshot = {"brand": {"title": "X"}}          # 快照缺一大堆键
+    merged = deep_merge(DEFAULT_CONFIG, snapshot)
+    assert merged["brand"]["title"] == "X"         # 覆盖生效
+    assert "color" in merged["brand"]              # 缺省补齐
+    assert "wall" in merged and "push" in merged   # 顶层组补齐
+
+
+def test_snapshot_not_mutated():
+    snapshot = {"brand": {"title": "X"}}
+    deep_merge(DEFAULT_CONFIG, snapshot)
+    assert snapshot == {"brand": {"title": "X"}}
+
+
+def test_nested_override_keeps_sibling():
+    snapshot = {"email": {"sender_name": "【T】"}}
+    merged = deep_merge(DEFAULT_CONFIG, snapshot)
+    assert merged["email"]["sender_name"] == "【T】"
+    assert "notify_emails" in merged["email"]      # 兄弟键保留默认
+
+
+# ---------- TenantConfig / Site 站点模型 ----------
+
+def test_site_overrides_and_inherits_tenant_brand():
+    """站点覆盖项生效；未覆盖项继承租户级 brand（而非出厂默认）。"""
+    tenant = TenantConfig.from_snapshot(
+        slug="t1",
+        name="Tenant 1",
+        snapshot={"brand": {"title": "T Co", "welcome": "hi from tenant"}},
+        sites=[
+            {"slug": "s1", "name": "Site 1", "domains": ["a.com"],
+             "brand": {"color": "#ff0000"}},
+        ],
+    )
+    site = tenant.sites[0]
+    assert isinstance(site, Site)
+    assert site.slug == "s1"
+    assert site.name == "Site 1"
+    assert site.domains == ("a.com",)
+    assert site.brand["color"] == "#ff0000"        # 站点覆盖生效
+    assert site.brand["title"] == "T Co"           # 继承租户级 brand
+    assert site.brand["welcome"] == "hi from tenant"
+    # 站点 brand 是独立副本，不与租户 data 共享可变对象
+    assert site.brand is not tenant.data["brand"]
+
+
+def test_tenant_without_sites():
+    """无 sites 时 sites 为空元组，data 正常深合并。"""
+    tenant = TenantConfig.from_snapshot(
+        slug="t2", name="Tenant 2", snapshot={"ai": {"model": "glm-4-plus"}},
+    )
+    assert tenant.sites == ()
+    assert tenant.status == "active"
+    assert tenant.data["ai"]["model"] == "glm-4-plus"       # 覆盖生效
+    assert tenant.data["push"]["enabled"] is False          # 缺省补齐