Fix 6 Postgres-specific test failures in workflow store

Root causes:
- PG store CreateInstance/CreateAssignment set defaults in local vars
  but didn't write them back to the struct, so callers saw empty
  Status fields (violating CHECK constraints on subsequent updates)
- PG JSONB normalizes whitespace ({"key": "val"} vs {"key":"val"})
  but tests compared exact strings
- ListSignoffs test used "nonexistent" as instance_id but PG
  validates UUID format

Fixes:
- Write defaults directly to inst.Status / a.Status in PG store
  (aligns with SQLite store which already did this)
- Add jsonEq() helper using json.Compact for whitespace-agnostic
  JSON comparison across all stage_data/review_data assertions
- Use valid zero-UUID for non-existent instance in signoff test

All 35 tests pass on SQLite (28 store + 7 engine).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/handlers/workflow_store_test.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"testing"
@@ -13,6 +14,19 @@ import (
 	"switchboard-core/store/sqlite"
 )
 
+// jsonEq compares two JSON byte slices ignoring whitespace differences
+// (PG JSONB normalizes spacing, SQLite preserves it verbatim).
+func jsonEq(a, b json.RawMessage) bool {
+	var ca, cb bytes.Buffer
+	if err := json.Compact(&ca, a); err != nil {
+		return false
+	}
+	if err := json.Compact(&cb, b); err != nil {
+		return false
+	}
+	return ca.String() == cb.String()
+}
+
 // testStores returns an appropriate Stores for the current dialect.
 func testStores(t *testing.T) store.Stores {
 	t.Helper()
@@ -128,7 +142,7 @@ func TestWorkflowInstance_CreateAndGet(t *testing.T) {
 	if got.EntryToken == nil || *got.EntryToken != token {
 		t.Errorf("entry_token = %v, want %q", got.EntryToken, token)
 	}
-	if string(got.StageData) != `{"key":"val"}` {
+	if !jsonEq(got.StageData, json.RawMessage(`{"key":"val"}`)) {
 		t.Errorf("stage_data = %s, want {\"key\":\"val\"}", got.StageData)
 	}
 	if got.CreatedAt.IsZero() {
@@ -200,8 +214,8 @@ func TestWorkflowInstance_Update(t *testing.T) {
 	if got.CurrentStage != stage2 {
 		t.Errorf("current_stage = %q, want %q", got.CurrentStage, stage2)
 	}
-	if string(got.StageData) != `{"updated":true}` {
-		t.Errorf("stage_data = %s", got.StageData)
+	if !jsonEq(got.StageData, json.RawMessage(`{"updated":true}`)) {
+		t.Errorf("stage_data = %s, want {\"updated\":true}", got.StageData)
 	}
 }
 
@@ -296,8 +310,8 @@ func TestWorkflowInstance_AdvanceStage(t *testing.T) {
 	if got.CurrentStage != stage2 {
 		t.Errorf("current_stage = %q, want %q", got.CurrentStage, stage2)
 	}
-	if string(got.StageData) != `{"step":"two"}` {
-		t.Errorf("stage_data = %s", got.StageData)
+	if !jsonEq(got.StageData, json.RawMessage(`{"step":"two"}`)) {
+		t.Errorf("stage_data = %s, want {\"step\":\"two\"}", got.StageData)
 	}
 	if !got.StageEnteredAt.After(originalEnteredAt) {
 		t.Errorf("stage_entered_at should have advanced; original=%v, got=%v",
@@ -543,8 +557,8 @@ func TestWorkflowAssignment_Complete(t *testing.T) {
 	if got.Status != models.AssignmentStatusCompleted {
 		t.Errorf("status = %q, want completed", got.Status)
 	}
-	if string(got.ReviewData) != `{"approved":true,"notes":"LGTM"}` {
-		t.Errorf("review_data = %s", got.ReviewData)
+	if !jsonEq(got.ReviewData, json.RawMessage(`{"approved":true,"notes":"LGTM"}`)) {
+		t.Errorf("review_data = %s, want {\"approved\":true,\"notes\":\"LGTM\"}", got.ReviewData)
 	}
 	if got.CompletedAt == nil {
 		t.Error("completed_at should be set")
@@ -837,7 +851,7 @@ func TestWorkflowSignoff_ListEmpty(t *testing.T) {
 	ctx := context.Background()
 
 	// ListSignoffs on non-existent instance should return empty, not error
-	list, err := s.Workflows.ListSignoffs(ctx, "nonexistent", "stage")
+	list, err := s.Workflows.ListSignoffs(ctx, "00000000-0000-0000-0000-000000000000", "stage")
 	if err != nil {
 		t.Fatalf("list signoffs error: %v", err)
 	}