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>

utils/anchoring.py

@@ -145,7 +145,7 @@ async def upgrade_ots_proof(proof_bytes: bytes) -> tuple[bytes, bool]:
     """
     # OpenTimestamps upgrade is done via the `ots` CLI or their calendar API.
     # For now, return the proof as-is with is_confirmed=False.
-    # TODO: Implement calendar-based upgrade polling.
+    # Calendar-based upgrade polling not yet implemented.
     return proof_bytes, False