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>

services/calendar_impl.py

@@ -371,7 +371,7 @@ class SqlCalendarService:
             entries_by_post.setdefault(post_id, []).append(_entry_to_dto(entry))
         return entries_by_post
 
-    # -- writes (absorb glue lifecycle) ---------------------------------------
+    # -- writes ---------------------------------------------------------------
 
     async def adopt_entries_for_user(
         self, session: AsyncSession, user_id: int, session_id: str,