feat(shared): cross-cutting plumbing and pure domain layer

Typed pydantic-settings config, structlog JSON logging with request/trace id
propagation and redaction of phone numbers, tokens and face keys, the unified
response envelope and error-code table from 架构文档 §9.2, async SQLAlchemy
session management, ORM models for the eight foundation tables, and the
OpenTelemetry/Prometheus wiring.

packages/domain stays pure logic -- snowflake ids, the quota three-phase
protocol and the tk_task state machine -- so it can be tested without a
database and is held to mypy strict.

packages/shared/context.py

@@ -0,0 +1,75 @@
+"""Request-scoped context propagated into logs and traces (架构文档 §15)."""
+
+from __future__ import annotations
+
+import uuid
+from contextvars import ContextVar, Token
+from dataclasses import dataclass
+
+_request_id: ContextVar[str | None] = ContextVar("meida_request_id", default=None)
+_trace_id: ContextVar[str | None] = ContextVar("meida_trace_id", default=None)
+_user_id: ContextVar[int | None] = ContextVar("meida_user_id", default=None)
+_task_id: ContextVar[int | None] = ContextVar("meida_task_id", default=None)
+
+
+@dataclass(slots=True)
+class ContextTokens:
+    request_id: Token[str | None]
+    trace_id: Token[str | None]
+    user_id: Token[int | None]
+    task_id: Token[int | None]
+
+
+def new_request_id() -> str:
+    return uuid.uuid4().hex
+
+
+def bind_context(
+    *,
+    request_id: str | None = None,
+    trace_id: str | None = None,
+    user_id: int | None = None,
+    task_id: int | None = None,
+) -> ContextTokens:
+    return ContextTokens(
+        request_id=_request_id.set(request_id if request_id is not None else _request_id.get()),
+        trace_id=_trace_id.set(trace_id if trace_id is not None else _trace_id.get()),
+        user_id=_user_id.set(user_id if user_id is not None else _user_id.get()),
+        task_id=_task_id.set(task_id if task_id is not None else _task_id.get()),
+    )
+
+
+def reset_context(tokens: ContextTokens) -> None:
+    _request_id.reset(tokens.request_id)
+    _trace_id.reset(tokens.trace_id)
+    _user_id.reset(tokens.user_id)
+    _task_id.reset(tokens.task_id)
+
+
+def get_request_id() -> str | None:
+    return _request_id.get()
+
+
+def get_trace_id() -> str | None:
+    return _trace_id.get()
+
+
+def get_user_id() -> int | None:
+    return _user_id.get()
+
+
+def get_task_id() -> int | None:
+    return _task_id.get()
+
+
+def context_snapshot() -> dict[str, str | int]:
+    snapshot: dict[str, str | int] = {}
+    for key, value in (
+        ("request_id", _request_id.get()),
+        ("trace_id", _trace_id.get()),
+        ("user_id", _user_id.get()),
+        ("task_id", _task_id.get()),
+    ):
+        if value is not None:
+            snapshot[key] = value
+    return snapshot