feat: M1 — 立项→写章草稿(SSE)→自动保存；连一家 provider

- 薄自建 LLM 网关：OpenAI 兼容适配器(DeepSeek) + instructor 结构化输出 + usage_ledger 记账 + 档位路由
- 记忆服务 assemble：确定性选择(显式+主角+近况) + 渲染卡 + 缓存断点(中性文本)
- LangGraph 写章节点 + Postgres checkpointer + SSE 归一(token/done/error)
- API：立项 + 写章 draft(SSE) + PUT 自动保存 + 提供商凭据(Fernet 加密/测试连接)
- 前端：AppShell + 作品库 + 5 步立项向导 + 写作工作台(流式打字机+自动保存) + 设置页
- M1 E2E：真实 DB + mock 网关零 token 走通闭环

apps/api/tests/fakes_providers.py

@@ -0,0 +1,56 @@
+"""内存替身：凭据存储 + 提供商探测（端点测试用，无 DB/无网络）。
+
+绝对导入 `from fakes import ...`（包目录无 __init__.py，见 memory/gotchas）。
+"""
+
+from __future__ import annotations
+
+import uuid
+
+from ww_api.services.credentials import StoredCredential, StoredRouting
+from ww_llm_gateway.adapters.base import Capabilities
+
+
+class FakeCredentialStore:
+    """实现 `CredentialStore` Protocol 的内存版。"""
+
+    def __init__(self) -> None:
+        # key: (owner_id, provider) → 密文
+        self.creds: dict[tuple[uuid.UUID, str], bytes] = {}
+        self.routing: dict[str, StoredRouting] = {}
+
+    async def list_credentials(self, owner_id: uuid.UUID) -> list[StoredCredential]:
+        return [
+            StoredCredential(provider=p, api_key_enc=blob)
+            for (o, p), blob in self.creds.items()
+            if o == owner_id
+        ]
+
+    async def list_routing(self) -> list[StoredRouting]:
+        return list(self.routing.values())
+
+    async def get_credential(self, owner_id: uuid.UUID, provider: str) -> StoredCredential | None:
+        blob = self.creds.get((owner_id, provider))
+        if blob is None:
+            return None
+        return StoredCredential(provider=provider, api_key_enc=blob)
+
+    async def upsert_credential(
+        self, owner_id: uuid.UUID, provider: str, api_key_enc: bytes
+    ) -> None:
+        self.creds[(owner_id, provider)] = api_key_enc
+
+    async def upsert_routing(self, routing: StoredRouting) -> None:
+        self.routing[routing.tier] = routing
+
+
+class FakeProviderProbe:
+    """实现 `ProviderProbe` Protocol：返回固定能力矩阵，绝不联网。"""
+
+    def __init__(self, caps: Capabilities | None = None) -> None:
+        self.caps = caps or Capabilities(structured_output=True, prefix_cache=True, thinking=False)
+        self.calls: list[tuple[uuid.UUID, str]] = []
+
+    async def probe(self, owner_id: uuid.UUID, provider: str) -> Capabilities:
+        self.calls.append((owner_id, provider))
+        return self.caps