chore(pm): vendor the Aramesh/PM platform under pm/ (squash of Aramesh/PM @ 91f3271)

The PM panel (PmPanelPage -> iframe /pm/) needs the project-management
platform served at /pm/ on the ERP domain. Source: org repo Aramesh/PM,
branch feature/project-management-platform. Update via:
git subtree pull --prefix=pm aramesh-pm feature/project-management-platform --squash

pm/backend/app/db.py

@@ -0,0 +1,20 @@
+"""Async SQLAlchemy engine/session for PM database."""
+from collections.abc import AsyncIterator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from app.config import settings
+
+try:
+    engine = create_async_engine(settings.DATABASE_URL, pool_pre_ping=True, pool_size=5, max_overflow=5)
+except Exception:
+    # Fallback for testing environments or when asyncpg driver is not installed on host
+    engine = create_async_engine("sqlite+aiosqlite:///:memory:")
+
+SessionFactory = async_sessionmaker(engine, expire_on_commit=False)
+
+
+
+async def get_db() -> AsyncIterator[AsyncSession]:
+    async with SessionFactory() as session:
+        yield session