Adds RoverPlanning.backfill_paths/0 that walks every mission and
re-enqueues RoverPathProfileWorker jobs for any (rover × station) pair
whose Path is missing or not :complete. The path-profile worker
short-circuits on :complete, so this is idempotent.
Wires a new RoverMissionBackfillWorker into both dev (config.exs) and
prod (runtime.exs) crons at HH:30 — heals missions stuck in :pending
or :failed from transient elevation-API errors or interrupted
create_mission enqueues without operator action.
