✨ feat: WP-05 模型网关核心——逻辑模型、流式适配与 Run 真实执行

- 迁移 7+8：logical_model（能力目录）、provider_endpoint（密钥只存环境变量名，永不落库）、turn、model_call（每 attempt 单独持久化：usage/TTFT/错误，Provider 未返回保持 null 不估算——ADR-003）；message.run_id 关联
- OpenAI-compatible 流式适配器：httpx SSE 解析、标准化 StreamChunk（delta/usage/done）、transport 可注入（测试与故障注入）；密钥经环境变量读取不进日志
- Run 执行器：后台执行完整生命周期——running → content.block.started → 真实 content.block.delta*（流式产生，delta_index 单调）→ content.block.completed → message.completed → 终态；assistant 消息为权威快照；ModelCall 记录 usage 与 TTFT；无可用端点时 Run 显式失败并留 error 事件（fail-closed）
- post_message：响应提交后由框架 BackgroundTasks 执行 Run（显式预提交保证 Run 落库）；SSE 改为增量轮询（重放 + 等待新事件至终态，30s 上限）
- 测试 54 项全绿（+2）：端到端 mock 流验证 ModelCall usage/TTFT/assistant 权威消息、无端点 Run failed + content.block.failed 事件

backend/app/models.py

@@ -207,6 +207,9 @@ class Message(Base):
         ForeignKey("message.id", ondelete="set null"), nullable=True
     )
     variant_kind: Mapped[str] = mapped_column(String, nullable=False, default="origin")
+    run_id: Mapped[str | None] = mapped_column(
+        ForeignKey("run.id", ondelete="set null"), nullable=True, index=True
+    )
     content: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False)
     parser_version: Mapped[str | None] = mapped_column(String, nullable=True)
     created_at: Mapped[datetime] = mapped_column(
@@ -374,3 +377,98 @@ class Character(Base):
             name="origin_values",
         ),
     )
+
+
+class LogicalModel(Base):
+    """逻辑模型：业务只引用能力与逻辑名，不感知 Provider（契约第 5 节分面）。"""
+
+    __tablename__ = "logical_model"
+
+    id: Mapped[str] = mapped_column(String, primary_key=True)  # kebab-case，如 default-chat
+    capabilities: Mapped[list[str]] = mapped_column(JSONB, nullable=False, default=list)
+    description: Mapped[str | None] = mapped_column(String, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+
+class ProviderEndpoint(Base):
+    """Provider 端点：密钥只存环境变量名，永不落库明文。"""
+
+    __tablename__ = "provider_endpoint"
+
+    id: Mapped[str] = mapped_column(
+        String, primary_key=True, default=lambda: new_id("endpoint_")
+    )
+    logical_model_id: Mapped[str] = mapped_column(
+        ForeignKey("logical_model.id", ondelete="cascade"), nullable=False, index=True
+    )
+    provider: Mapped[str] = mapped_column(String, nullable=False)
+    name: Mapped[str] = mapped_column(String, nullable=False)
+    base_url: Mapped[str] = mapped_column(String, nullable=False)
+    model_id: Mapped[str] = mapped_column(String, nullable=False)
+    api_key_env: Mapped[str] = mapped_column(String, nullable=False)
+    priority: Mapped[int] = mapped_column(nullable=False, default=100)
+    enabled: Mapped[bool] = mapped_column(nullable=False, default=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+    __table_args__ = (
+        CheckConstraint(
+            "provider IN ('openai_compatible', 'anthropic')", name="provider_values"
+        ),
+    )
+
+
+class Turn(Base):
+    """Turn：Run 内一次模型决策及其工具交互阶段。"""
+
+    __tablename__ = "turn"
+
+    id: Mapped[str] = mapped_column(String, primary_key=True, default=lambda: new_id("turn_"))
+    run_id: Mapped[str] = mapped_column(
+        ForeignKey("run.id", ondelete="cascade"), nullable=False, index=True
+    )
+    index: Mapped[int] = mapped_column(nullable=False, default=0)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+    __table_args__ = (
+        Index("uq_turn_run_index", "run_id", "index", unique=True),
+    )
+
+
+class ModelCall(Base):
+    """每次上游 attempt 单独持久化：重试/故障转移不覆盖（ADR-003）。
+
+    Provider 未返回的 usage 字段保持 null，不估算伪装。
+    """
+
+    __tablename__ = "model_call"
+
+    id: Mapped[str] = mapped_column(
+        String, primary_key=True, default=lambda: new_id("model_call_")
+    )
+    turn_id: Mapped[str] = mapped_column(
+        ForeignKey("turn.id", ondelete="cascade"), nullable=False, index=True
+    )
+    endpoint_id: Mapped[str | None] = mapped_column(
+        ForeignKey("provider_endpoint.id", ondelete="set null"), nullable=True
+    )
+    attempt: Mapped[int] = mapped_column(nullable=False, default=1)
+    status: Mapped[str] = mapped_column(String, nullable=False, default="pending")
+    request_summary: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False, default=dict)
+    usage: Mapped[dict[str, Any] | None] = mapped_column(JSONB, nullable=True)
+    ttft_ms: Mapped[int | None] = mapped_column(nullable=True)
+    error: Mapped[str | None] = mapped_column(String, nullable=True)
+    started_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    completed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+
+    __table_args__ = (
+        CheckConstraint(
+            "status IN ('pending', 'streaming', 'succeeded', 'failed')", name="status_values"
+        ),
+        Index("uq_model_call_turn_attempt", "turn_id", "attempt", unique=True),
+    )