Feat v0.3.2 workflow engine + handlers

Workflow execution engine with Start/Advance/Cancel lifecycle, automated
stage processor with Starlark hook firing and cycle guard (max 10),
instance and assignment HTTP handlers, store round-trip tests for all
v0.3.1 methods, and Starlark module expansion (get_instance, list_instances).

New routes:
- POST/GET /workflows/:id/instances (start, list)
- GET /workflows/:id/instances/:iid (get)
- POST /workflows/:id/instances/:iid/advance (advance)
- POST /workflows/:id/instances/:iid/cancel (cancel)
- POST /assignments/:id/claim|unclaim|complete|cancel
- GET /assignments/mine
- Team-scoped mirrors under /teams/:teamId/...

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

server/database/testhelper.go

@@ -256,7 +256,9 @@ func TruncateAll(t *testing.T) {
 		// HA
 		"rate_limit_counters",
 		"ws_tickets",
-		// Workflows
+		// Workflows (FK order: assignments → instances → versions → stages → workflows)
+		"workflow_assignments",
+		"workflow_instances",
 		"workflow_versions",
 		"workflow_stages",
 		"workflows",