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>

app/platform/tenant_config.py

@@ -0,0 +1,35 @@
+"""租户与站点配置模型。
+
+- TenantConfig.data = deep_merge(DEFAULT_CONFIG, snapshot)：租户快照 ⊕ 出厂默认
+- Site.brand 的合并基线是**租户级** data["brand"]（而非出厂默认），
+  这样租户级 brand 自定义也能被各站点继承。
+"""
+from dataclasses import dataclass
+
+from app.platform.defaults import DEFAULT_CONFIG, deep_merge
+
+
+@dataclass(frozen=True)
+class Site:
+    slug: str
+    name: str
+    domains: tuple[str, ...]
+    brand: dict                 # 站点门面覆盖层 ⊕ 租户默认 brand（已深合并）
+
+
+@dataclass(frozen=True)
+class TenantConfig:
+    slug: str
+    name: str
+    status: str                 # active | disabled
+    data: dict                  # deep_merge(DEFAULT_CONFIG, snapshot)
+    sites: tuple[Site, ...] = ()
+
+    @classmethod
+    def from_snapshot(cls, slug, name, snapshot, sites=(), status="active"):
+        merged = deep_merge(DEFAULT_CONFIG, snapshot)
+        resolved = tuple(
+            Site(s["slug"], s.get("name", s["slug"]), tuple(s.get("domains", ())),
+                 deep_merge(merged["brand"], s.get("brand", {})))
+            for s in sites)
+        return cls(slug=slug, name=name, status=status, data=merged, sites=resolved)