fix: create user_binary row on character creation (#176)

New characters were missing their user_binary record, preventing them
from entering their house. Both sign server and API character creation
paths now insert the row. A backfill migration fixes existing databases.

server/migrations/sql/0006_backfill_user_binary.sql

@@ -0,0 +1,6 @@
+-- Backfill user_binary rows for characters that were created without one.
+-- This fixes #176: new characters could not enter their house.
+INSERT INTO user_binary (id)
+SELECT c.id FROM characters c
+LEFT JOIN user_binary ub ON ub.id = c.id
+WHERE ub.id IS NULL;