feat(pres_mgmt): restore location and description editing in session view

Event location (FK lookup) and description were both visible in the session
view but had no edit controls — lost during V3 migration. Restored both:

- event_location_id: select dropdown populated from this event's location list
  (liveQuery on db_events.location filtered by event_id from the session object)
- description: textarea editor shown directly in edit_mode (no collapse needed
  when actively editing)

Also added event_location_id to editable_fields__event_session, which was
missing and would have caused backend rejections on PATCH.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/ae_events/ae_events__event_session.editable_fields.ts

@@ -2,6 +2,7 @@ export const editable_fields__event_session = [
     'external_id',
     'code',
     'type_code',
+    'event_location_id',
     'poc_agree',
     'poc_kv_json',
     'name',