perf: eliminate N+1 queries in __make_cre_links and __get_all_nodes_and_cres

[shiwani42]

Fixes #848

What changed

__make_cre_links

Replaced the per-link Node fetch with a single JOIN query. Previously, for a CRE with L links this method issued 1 + L queries. It now issues one query regardless of link count.

Before:

for link in self.session.query(Links).filter(Links.cre == cre.id).all():
    node = self.session.query(Node).filter(Node.id == link.node).first()

After:

rows = (
    self.session.query(Links, Node)
    .join(Node, Node.id == Links.node)
    .filter(Links.cre == cre.id)
    .all()
)
for link, node in rows:
    ...

__get_all_nodes_and_cres

Replaced the IDs-first fetch pattern with direct ORM object queries. For CREs, the previous code called get_cre_by_db_id per record, which added a SELECT external_id FROM cre WHERE id = ? round-trip before reaching get_CREs. Fetching full ORM objects and passing the already-known external_id directly cuts that out. The same fix is applied to the node branch.

Tests

All existing tests pass. The changed methods are exercised by test_get_CREs, test_export, test_get_root_cres, and test_get_cre_hierarchy.