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>

alembic/env.py

@@ -19,7 +19,7 @@ from shared.db.base import Base
 
 # Import ALL models so Base.metadata sees every table
 import shared.models  # noqa: F401  User, KV, MagicLink, MenuItem, Ghost*
-for _mod in ("blog.models", "market.models", "cart.models", "events.models", "federation.models", "glue.models"):
+for _mod in ("blog.models", "market.models", "cart.models", "events.models", "federation.models"):
     try:
         __import__(_mod)
     except ImportError: