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/__init__.py

@@ -0,0 +1,2 @@
+"""平台层（多租户）：出厂默认、配置合并、租户/站点模型、注册表等。
+Phase 1 逐步迁入；本包不得反向依赖 app/engine 之外的假设。"""

app/platform/defaults.py

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+"""出厂默认配置。结构 = spec §5.1 的 9 个配置组。
+本阶段 brand/wall 的默认值由 Task 4 从 jc 代码提取填充。"""
+import copy
+
+DEFAULT_CONFIG: dict = {
+    "brand": {
+        "title": "AI Support", "color": "#066fc0", "avatar": "/static/kefu-small.png",
+        "welcome": "", "topics": [], "placeholder": "",
+        "whatsapp": "", "email": "", "privacy_url": "",
+    },
+    "wall": {},   # Task 4 填充
+    "contact": {"cc_completion": True, "email_light_ask": True, "template_languages": ["en", "zh", "es"]},
+    "email": {"smtp": None, "sender_name": "", "notify_emails": []},
+    "push": {"enabled": False, "url": "", "api_key": "", "prefix": "", "timeout": 10,
+             "max_attempts": 3, "retry_backoff_base": 1.0},
+    "ai": {"provider": "glm", "api_key": None, "base_url": None, "model": "glm-4-flash"},
+    "engine": {"context_seed_keys": {}, "rate_limit_per_hour": 30},
+    "misc": {"name_to_region_extras": {}},
+}
+
+
+def deep_merge(base: dict, override: dict) -> dict:
+    """override 逐层覆盖 base；dict 递归合并，其余类型整体替换。不修改入参。"""
+    out = copy.deepcopy(base)
+    for k, v in (override or {}).items():
+        if isinstance(v, dict) and isinstance(out.get(k), dict):
+            out[k] = deep_merge(out[k], v)
+        else:
+            out[k] = copy.deepcopy(v)
+    return out

app/platform/tenant_config.py

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+"""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)