feat(sbc): make submissions atomic and durable

migrations/0021_sbc_challenge_squads.sql

@@ -0,0 +1,9 @@
+-- Core owns durable per-profile working squads for SBC challenges. The FIFA17
+-- adapter maps its numeric challenge id to the opaque generic sbc_id.
+CREATE TABLE sbc_challenge_squads (
+    profile_id TEXT NOT NULL REFERENCES profiles(id),
+    sbc_id TEXT NOT NULL,
+    owned_card_ids TEXT NOT NULL,
+    updated_at TEXT NOT NULL,
+    PRIMARY KEY (profile_id, sbc_id)
+);