feat: core ORM models + initial migration

Implements the 14 core ORM models for the yunke data-mirror system using
SQLAlchemy 2.0 Mapped/mapped_column style, driven by the field lists in
docs/superpowers/specs/2026-08-01-yunke-data-dictionary.md:

  Department, Staff, WechatAccount, WhatsappAccount, Customer, ChatGroup,
  GroupMember, Message, Media, PhoneCall, Sms, SyncState, YkAccount, SyncRun.

Conventions:
- Natural source PKs where they exist (friend_id, wx_id, msg_id, room_id,
  media_hash, call_id, sms_id, dept_id, user_id); surrogate Integer PKs
  otherwise (SyncState, YkAccount, SyncRun). GroupMember uses a composite
  (channel, room_id, member_id) PK.
- Common audit columns version/raw_json/fetched_at/updated_at on every
  source-mirrored table.
- channel only where meaningful: Customer/Message/Media/ChatGroup/
  GroupMember/Sms use messaging channels; PhoneCall.channel is call-type;
  SyncState.channel scopes the cursor. Omitted on Staff/Department/
  WechatAccount/WhatsappAccount/YkAccount/SyncRun.
- Message composite index on (channel, owner_account_id, talker_id,
  timestamp) for the hot per-talker time-order query path.
- message_table_name(channel, timestamp) helper documents the future
  message_<channel>_<YYYYMM> sharding convention; the Message model still
  maps to a single physical table for now (sharding is a later task).

All types are portable generic SQLAlchemy types so tests run on in-memory
SQLite while prod targets MySQL. Tests verify Customer round-trip,
raw_json JSON round-trip, Message natural PK, GroupMember composite PK,
the Message composite index, and full table creation on SQLite.

Alembic wired via migrations/env.py to Base.metadata + get_settings().db_url
with an async->sync driver swap (mysql+asyncmy -> mysql+pymysql,
sqlite+aiosqlite -> sqlite). Initial migration autogenerated against an
empty SQLite DB; server defaults normalized to sa.func.now() so the file
is MySQL-portable. upgrade head is intentionally not run (no MySQL yet).

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

yunke-server/pyproject.toml

@@ -8,6 +8,10 @@ dependencies = [
     "uvicorn[standard]",
     "sqlalchemy[asyncio]",
     "asyncmy",
+    # Pure-Python sync MySQL driver used by Alembic migrations (env.py swaps
+    # the production ``mysql+asyncmy`` URL to ``mysql+pymysql`` for the sync
+    # migration engine).
+    "pymysql",
     "alembic",
     "redis",
     "httpx",