feat(sbc): make submissions atomic and durable

migrations/0020_atomic_sbc_submissions.sql

@@ -0,0 +1,14 @@
+-- Durable replay and non-repeatable-completion guards for atomic SBC submissions.
+-- Existing successful rows are treated as non-repeatable; if historical data already
+-- violates that invariant the migration fails rather than silently discarding history.
+ALTER TABLE sbc_submissions ADD COLUMN repeatable INTEGER NOT NULL DEFAULT 0;
+
+CREATE UNIQUE INDEX idx_sbc_nonrepeatable_completion
+ON sbc_submissions(profile_id, sbc_id)
+WHERE passed = 1 AND repeatable = 0;
+
+-- submitted_card_ids is stored in canonical sorted order by the writer. This rejects
+-- stale retries of the same card set even for explicitly repeatable challenges.
+CREATE UNIQUE INDEX idx_sbc_submission_replay
+ON sbc_submissions(profile_id, sbc_id, submitted_card_ids)
+WHERE passed = 1;