Add a stock ledger, inter-branch transfers and a sync protocol that cannot jam

Stock (PRD 5.2): a branch's stock of a product is now the sum of its
StockMovement rows. Every movement names the document that caused it and
(type, refId) is unique, so replaying a sale, retrying a transfer or a
second M-Pesa callback never moves the same stock twice.

- Cash sales and confirmed M-Pesa payments deduct stock; purchases of a
  catalogue product add it.
- POST /api/stock/transfers moves stock between two branches (owner: any
  two; manager: out of their own) and refuses to send more than the
  sending branch holds. Double submits are applied once.
- POST /api/stock/adjustments records opening stock, breakage or a
  physical count (the ledger is corrected to the counted number).
- GET /api/stock returns every branch x product with LOW/OUT flags
  (per-product lowStockThreshold, default 10); GET /api/stock/movements
  is the history.

Sync (PRD 5.3/5.5): POST /api/transactions/cash now returns a result per
sale - saved, duplicate or rejected - instead of silently skipping
replays. The old client never learned a replayed sale had synced, so
rows piled up until the batch passed the 100-sale limit and every flush
failed with 400. Also:
- A sale can hold several products (one Transaction per line, grouped by
  saleId); the old single-product payload is still accepted.
- One bad sale is rejected on its own instead of failing the batch.
- A queued sale is only accepted from the cashier who recorded it, and
  stays with the branch where it happened.
- A product deactivated while offline is still recorded at its price; an
  invalid phone is dropped rather than losing the sale; the phone is now
  optional for cash sales.
- The price the device showed is kept and a difference is flagged
  (priceMismatch) instead of being silently overwritten.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/server/models/Transaction.ts

@@ -9,7 +9,7 @@ export interface ITransaction extends Document {
   branchId: Types.ObjectId;
   staffId: Types.ObjectId;
   customerName: string | null;
-  customerPhone: string;
+  customerPhone: string | null; // optional for cash sales; required for M-Pesa
   productId: Types.ObjectId;
   sizeLiters: number; // snapshotted at sale time
   quantity: number;
@@ -26,8 +26,15 @@ export interface ITransaction extends Document {
     resultDesc: string | null;
   };
 
-  clientUuid: string; // idempotency key generated on-device
+  clientUuid: string; // idempotency key for this line: `${saleId}:${lineIndex}` for cash sales
+  saleId: string | null; // groups the lines of one multi-item sale (the device's sale UUID)
+  lineIndex: number;
   syncedFromOffline: boolean;
+  // What the device showed and collected per unit. The server price stays
+  // authoritative; a difference (e.g. the price changed while the device
+  // was offline) is kept and flagged rather than silently overwritten.
+  clientUnitPrice: number | null;
+  priceMismatch: boolean;
 
   // TAMPER PREVENTION: this is what the device *claimed* the time was when
   // the sale happened — informational only, e.g. for offline-queue
@@ -50,7 +57,7 @@ const transactionSchema = new Schema<ITransaction>(
     branchId: { type: Schema.Types.ObjectId, ref: "Branch", required: true },
     staffId: { type: Schema.Types.ObjectId, ref: "User", required: true },
     customerName: { type: String, default: null, trim: true },
-    customerPhone: { type: String, required: true },
+    customerPhone: { type: String, default: null },
     productId: { type: Schema.Types.ObjectId, ref: "Product", required: true },
     sizeLiters: { type: Number, required: true },
     quantity: { type: Number, required: true, min: 1 },
@@ -73,7 +80,11 @@ const transactionSchema = new Schema<ITransaction>(
     },
 
     clientUuid: { type: String, required: true },
+    saleId: { type: String, default: null },
+    lineIndex: { type: Number, default: 0 },
     syncedFromOffline: { type: Boolean, default: false },
+    clientUnitPrice: { type: Number, default: null },
+    priceMismatch: { type: Boolean, default: false },
     clientSubmittedAt: { type: Date, default: null },
   },
   { timestamps: true } // createdAt/updatedAt set from the server's Date.now(), not client input
@@ -92,6 +103,7 @@ transactionSchema.index({ clientUuid: 1 }, { unique: true });
 transactionSchema.index({ branchId: 1, createdAt: -1 });
 transactionSchema.index({ businessId: 1, createdAt: -1 });
 transactionSchema.index({ paymentStatus: 1 });
+transactionSchema.index({ businessId: 1, saleId: 1 });
 
 export const Transaction =
   (models.Transaction as Model<ITransaction>) ||