Add a screen to set prices per branch

Branch price overrides were already applied at the till, in cash sales
and in M-Pesa, but nothing could set them.

- Products -> Branch prices (/admin/prices): pick a branch and set its
  price for any product; setting it back to the default removes the
  override, and "Use default" resets it. Branch managers see their own
  branch's prices read-only.
- /api/branch-prices: GET (owner: any branch; manager: own), PUT and
  DELETE (owner only). Branch and product must belong to the business;
  prices must be above zero; who changed a price is recorded.
- Branch pricing is a plan feature, as the Growth plan already
  advertised: Growth and Enterprise, plus any business on its free
  trial. A paid Starter business keeps any prices set during its trial
  and can reset them, but cannot set new ones.
- The catalogue view on the Products page always shows default prices;
  the till's view shows what the branch charges.

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

src/lib/server/models/Product.ts

@@ -41,6 +41,7 @@ export interface IBranchPricing extends Document {
   branchId: Types.ObjectId;
   productId: Types.ObjectId;
   unitPrice: number;
+  updatedBy: Types.ObjectId | null;
   updatedAt: Date;
 }
 
@@ -50,6 +51,7 @@ const branchPricingSchema = new Schema<IBranchPricing>(
     branchId: { type: Schema.Types.ObjectId, ref: "Branch", required: true },
     productId: { type: Schema.Types.ObjectId, ref: "Product", required: true },
     unitPrice: { type: Number, required: true, min: 0 },
+    updatedBy: { type: Schema.Types.ObjectId, ref: "User", default: null },
   },
   { timestamps: { createdAt: false, updatedAt: true } }
 );

src/lib/tiers.ts

@@ -13,6 +13,7 @@ export interface TierDef {
   priceKsh: number; // monthly price in KSh
   maxBranches: number | null; // null = unlimited
   maxUsers: number | null; // total users in the business (owner + managers + staff); null = unlimited
+  branchPricing: boolean; // may set a different price per branch
   features: string[];
 }
 
@@ -23,6 +24,7 @@ export const TIERS: Record<TierId, TierDef> = {
     priceKsh: 3500,
     maxBranches: 1,
     maxUsers: 6, // owner + up to 5 staff/manager
+    branchPricing: false,
     features: [
       "1 branch",
       "Up to 5 staff accounts",
@@ -37,6 +39,7 @@ export const TIERS: Record<TierId, TierDef> = {
     priceKsh: 7500,
     maxBranches: 5,
     maxUsers: 30,
+    branchPricing: true,
     features: [
       "Up to 5 branches",
       "Up to 30 staff accounts",
@@ -51,6 +54,7 @@ export const TIERS: Record<TierId, TierDef> = {
     priceKsh: 15000,
     maxBranches: null,
     maxUsers: null,
+    branchPricing: true,
     features: [
       "Unlimited branches",
       "Unlimited staff accounts",
@@ -72,6 +76,15 @@ export const TRIAL_DAYS = 14;
 // applies to adding branches; existing branches are never removed.
 export const TRIAL_MAX_BRANCHES = 2;
 
+/**
+ * Whether the business may set branch-specific prices right now: a plan
+ * feature (Growth and up), also available during the free trial so it can
+ * be tried with the trial's two branches.
+ */
+export function canUseBranchPricing(tier: TierId, status: SubscriptionStatus): boolean {
+  return TIERS[tier].branchPricing || status === "TRIALING";
+}
+
 /** Branch limit for a business right now (null = unlimited). */
 export function effectiveMaxBranches(tier: TierId, status: SubscriptionStatus): number | null {
   const max = TIERS[tier].maxBranches;