feat(server): 试运行性能索引补强（E9）

- reimbursement_forms/invoice_pool/workflow_tasks/audit_logs 增加常用筛选组合索引
- 仅新增索引与迁移，便于回滚

server/prisma/migrations/20260208155029_p4_index_optimizations/migration.sql

@@ -0,0 +1,20 @@
+-- DropIndex
+DROP INDEX "idx_audit_logs_query";
+
+-- CreateIndex
+CREATE INDEX "idx_audit_logs_query" ON "audit_logs"("created_at", "actor_id", "action");
+
+-- CreateIndex
+CREATE INDEX "idx_audit_logs_target_type_created_at" ON "audit_logs"("target_type", "created_at");
+
+-- CreateIndex
+CREATE INDEX "idx_invoice_pool_owner_status_created_at" ON "invoice_pool"("owner_employee_id", "status", "created_at");
+
+-- CreateIndex
+CREATE INDEX "idx_reimbursement_owner_status_updated_at" ON "reimbursement_forms"("owner_id", "status", "updated_at");
+
+-- CreateIndex
+CREATE INDEX "idx_workflow_tasks_status_assignee_id_created_at" ON "workflow_tasks"("status", "assignee_id", "created_at");
+
+-- CreateIndex
+CREATE INDEX "idx_workflow_tasks_status_assignee_role_created_at" ON "workflow_tasks"("status", "assignee_role", "created_at");