feat(server): 完成 P2 后端能力（支付幂等、委派回收、OCR异步、审计）

server/prisma/migrations/20260207180145_p2_phase1/migration.sql

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+-- reimbursement_forms: payment voucher fields
+ALTER TABLE "reimbursement_forms"
+  ADD COLUMN "payment_voucher_no" VARCHAR(50),
+  ADD COLUMN "payment_date" DATE,
+  ADD COLUMN "payment_channel" VARCHAR(20),
+  ADD COLUMN "paid_at" TIMESTAMP(3),
+  ADD COLUMN "paid_by" UUID;
+
+ALTER TABLE "reimbursement_forms"
+  ADD CONSTRAINT "reimbursement_forms_paid_by_fkey"
+  FOREIGN KEY ("paid_by") REFERENCES "employees"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+CREATE INDEX "reimbursement_forms_paid_by_idx" ON "reimbursement_forms"("paid_by");
+
+-- invoice_pool: OCR lock fields
+ALTER TABLE "invoice_pool"
+  ADD COLUMN "ocr_locked_at" TIMESTAMP(3),
+  ADD COLUMN "ocr_lock_owner" VARCHAR(128);
+
+CREATE INDEX "idx_invoice_pool_ocr_queue"
+ON "invoice_pool"("status", "ocr_locked_at", "created_at");
+
+-- audit_logs: query acceleration
+CREATE INDEX "idx_audit_logs_query"
+ON "audit_logs"("created_at" DESC, "actor_id", "action");