Reads an exported mtg_state_v4 JSON backup (the
kind Deck Hub's Import/Export produces) and loads it into the real Supabase
schema: players, decks, games and their seats. Safe to run more than once —
anything already imported (matched by name for players, or by an internal
legacy_id tag for decks/games) is skipped, not duplicated.
Elo will read as 1200 for every deck after this runs — that's expected, not a bug. Elo is Deck Hub-owned and always recomputed from scratch by replaying the full game log; that replay logic doesn't exist against Postgres yet (it's part of Phase 4, porting the Deck Hub itself). This importer only loads the raw game history it needs to replay later — it deliberately does not try to reconstruct Elo numbers itself, so there's only ever one place that math lives.
Shows what would happen — nothing is written yet.