hotfix(0031): DROP IF EXISTS before ADD (constraint pre-exists on PROD)

alembic/versions/20260528_0031_outcome_requires_inference_check.py

@@ -37,21 +37,34 @@
 
 
 def upgrade() -> None:
-    # Postgres CHECK constraints can't be DEFERRABLE (only FK/UNIQUE/PK/EXCLUDE
-    # support deferral). The two-phase outcome write (insert_decision phase
-    # writes the row with decision_rule='cheapest_clearing_floor' and
-    # inference_id=NULL; complete_decision phase links inference_id) means a
-    # naive `decision_rule <> 'cheapest_clearing_floor' OR inference_id IS NOT NULL`
-    # would fire on the initial insert.
+    # 2026-05-28 PROD note: the human+Claude repair session earlier today
+    # already added a constraint named
+    # `outcome_requires_inference_when_model_chosen` with an OLDER predicate
+    # (`decision_rule <> 'cheapest_clearing_floor' OR inference_id IS NOT NULL`)
+    # — that predicate breaks the two-phase outcome write because it fires at
+    # statement time (PG CHECK can't be DEFERRABLE). We DROP it and re-ADD
+    # with the new predicate (which adds `outcome_status IS NULL` as the
+    # mid-flight escape clause).
     #
-    # Mitigation: the predicate also allows `outcome_status IS NULL` — the
-    # marker for "this outcome is mid-flight, complete_decision hasn't run
-    # yet". Once complete_decision sets outcome_status (always non-NULL on
-    # every terminal branch — succeeded / failed_other / failed_provider_error
-    # / rejected_floor / rejected_no_candidates), the constraint REQUIRES
-    # either (a) the rule was rewritten away from 'cheapest_clearing_floor'
-    # (pre-dispatch failure → 'failed_pre_dispatch'), or (b) the inference is
-    # linked. Which is exactly the AIN-300 W1 invariant.
+    # Idempotent: DROP IF EXISTS so re-running the migration on a fresh DB
+    # (no prior constraint) is also clean.
+    op.execute(
+        """
+        ALTER TABLE public.routing_outcomes
+          DROP CONSTRAINT IF EXISTS outcome_requires_inference_when_model_chosen;
+        """
+    )
+
+    # Predicate explanation:
+    #   - `outcome_status IS NULL`              → row is mid-flight (insert_decision
+    #                                              just ran; complete_decision hasn't)
+    #   - `decision_rule <> 'cheapest_clearing_floor'`  → no model was chosen
+    #                                              (reject path / pre-dispatch fail)
+    #   - `inference_id IS NOT NULL`            → linkage holds
+    # By commit time of any terminal branch, outcome_status is non-NULL AND
+    # either decision_rule was rewritten via decision_rule_override OR
+    # inference_id is linked. So the constraint REQUIRES the AIN-300 W1
+    # invariant on committed rows.
     op.execute(
         """
         ALTER TABLE public.routing_outcomes