feat: session login + authed client

Task 1.4 — 会话层. Adds the authenticated HTTP client that every yunke
scraper will use to call the CRM API.

- app/session/login.py: get_secure_key + pc_login wrappers, LoginResult
  dataclass, and a clearly-marked Phase-0 swappable encrypt_password
  placeholder (real RSA per getSecureKey is a TODO; drop-in contract
  documented in the docstring).
- app/session/yunke_client.py: YunkeClient wrapping httpx.AsyncClient.
  Constructor takes either (username+password) for auto-login or
  (userid+company[+cookies]) for an existing session. Injects the 5
  observed auth headers (userid/company/source=yunkepc/channel=4/
  content-type=application/json) on every request, auto-logs-in on first
  request, and on HTTP 401 (or body code 401) re-logs-in once and
  retries — a persistent 401 raises after the single retry so there is
  no infinite loop.
- Tests (respx): login populates LoginResult + httpx cookie jar; every
  request carries all 5 auth headers + cookie; 401 triggers exactly one
  re-login + retry and returns 200; persistent 401 raises after one
  retry; first request with no session auto-logs-in.

Session is held in-memory only for now; persistence to encrypted
YkAccount.session_blob_enc is left as a later concern (TODO in
yunke_client.py).

Co-Authored-By: Claude <noreply@anthropic.com>

yunke-server/app/session/__init__.py

@@ -0,0 +1,29 @@
+"""会话层 (session layer) for the yunke CRM API.
+
+Public surface:
+
+* :class:`app.session.login.LoginResult` — parsed login outcome (userid,
+  company, cookies snapshot, raw payload).
+* :func:`app.session.login.get_secure_key` — ``GET /newuc/login/getSecureKey``.
+* :func:`app.session.login.pc_login` — ``POST /usercenter/login/pcLogin``.
+* :func:`app.session.login.encrypt_password` — Phase-0 swappable password
+  encryption (see file for the open TODO).
+* :class:`app.session.yunke_client.YunkeClient` — the authenticated HTTP
+  client every scraper uses.
+"""
+
+from app.session.login import (
+    LoginResult,
+    encrypt_password,
+    get_secure_key,
+    pc_login,
+)
+from app.session.yunke_client import YunkeClient
+
+__all__ = [
+    "LoginResult",
+    "YunkeClient",
+    "encrypt_password",
+    "get_secure_key",
+    "pc_login",
+]