Tech debt cleanup: fix session.py, remove stale references, update docs

- db/session.py: fix indentation (2→4 space), pool_size=0 (unlimited),
  remove "ned to look at this" typo
- Remove glue.models from alembic env.py import list
- Update shared __init__.py, menu_item.py docstring, calendar_impl.py,
  handlers/__init__.py to remove glue terminology
- Remove federation_handlers.py tombstone file
- Remove TODO comments (replace with explanatory comments)
- Rewrite README.md to reflect current architecture
- Update anchoring.py TODO to plain comment

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/menu_item.py

@@ -6,7 +6,7 @@ from shared.db.base import Base
 
 
 class MenuItem(Base):
-    """Deprecated — kept so the table isn't dropped. Use glue.models.MenuNode."""
+    """Deprecated — kept so the table isn't dropped. Use shared.models.menu_node.MenuNode."""
     __tablename__ = "menu_items"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)