docs(backend): update db.py docstring to reflect migrations-first order

backend/src/cyclone/db.py

@@ -2,9 +2,10 @@
 
 `init_db()` is called from `cyclone.__main__:serve` before uvicorn starts.
 It reads `CYCLONE_DB_URL` (default `sqlite:///~/.local/share/cyclone/cyclone.db`),
-creates the engine, runs `Base.metadata.create_all`, then runs the migration
-runner. Calling `init_db()` is idempotent — calling it twice on the same URL
-re-uses the same engine.
+creates the engine, runs the migration runner, then runs `Base.metadata.create_all`
+as a safety net (which is a no-op when the migration is current). Calling
+`init_db()` is idempotent — calling it twice on the same URL re-uses the
+same engine.
 """
 
 from __future__ import annotations