Promote Missing Key

Promote missing-events that carry a REAL default value into project keys (task 745).

For each event with has_default_value, create the project key (in the production version) with the captured default as its source-language value — which auto-resolves the open event (via _ensure_key → resolve_open_missing). Events without a real default error per item (no source to seed); unknown / cross-project uuids and vanished namespaces also error per item. Idempotent: an already-existing key returns created=false (and still resolves the event). The plan key-cap is a whole-batch gate (mirrors create_keys_bulk).