feat(platform-service): add getRequestProductId() + getRequestProductConfig() helpers

- New lib/request-context.ts with product validation against cache
- Priority: JWT payload > X-Product-Id header > env var fallback
- Rejects unknown or disabled products with 400 Bad Request
- Augments FastifyRequest with jwtPayload type declaration
- getRequestProductConfig() for modules needing product-specific values

services/platform-service/src/lib/request-context.ts

@@ -0,0 +1,71 @@
+/**
+ * Request-level product context helpers.
+ *
+ * Extract and validate productId from every request:
+ *   1. JWT token payload (authenticated requests)
+ *   2. X-Product-Id header (unauthenticated requests)
+ *   3. PRODUCT_ID env var fallback (backward compat during migration)
+ *
+ * Validates against the products registry — rejects unknown or disabled products.
+ */
+
+import type { FastifyRequest } from 'fastify';
+import { BadRequestError } from './errors.js';
+import { isValidProduct, getProduct } from '../modules/products/cache.js';
+import type { ProductDoc } from '../modules/products/types.js';
+
+/** JWT payload shape attached to req by the onRequest hook (see Commit 3). */
+export interface JwtPayload {
+  sub: string;
+  email?: string;
+  role?: string;
+  productId?: string;
+  type?: string;
+}
+
+// Augment Fastify request to include parsed JWT payload
+declare module 'fastify' {
+  interface FastifyRequest {
+    jwtPayload?: JwtPayload;
+  }
+}
+
+/**
+ * Extract and validate productId from the request.
+ * Priority: JWT token > X-Product-Id header > env fallback (dev only).
+ * Rejects unknown or disabled products.
+ */
+export function getRequestProductId(req: FastifyRequest): string {
+  // 1. From JWT (set during login/register, attached by onRequest hook)
+  let id = req.jwtPayload?.productId;
+
+  // 2. From header (unauthenticated requests like license activation)
+  if (!id) {
+    const header = req.headers['x-product-id'];
+    if (typeof header === 'string' && header.length > 0) id = header;
+  }
+
+  // 3. Fallback to env var (backward compat during migration, dev only)
+  if (!id) {
+    const envFallback = process.env.PRODUCT_ID;
+    if (envFallback) id = envFallback;
+  }
+
+  if (!id) throw new BadRequestError('productId is required (via JWT or X-Product-Id header)');
+
+  // Validate against product registry
+  if (!isValidProduct(id)) throw new BadRequestError(`Unknown product: ${id}`);
+  const product = getProduct(id)!;
+  if (product.status === 'disabled') throw new BadRequestError(`Product ${id} is disabled`);
+
+  return id;
+}
+
+/**
+ * Get the full product config for the current request's productId.
+ * Useful when modules need product-specific values (trial days, device limits, etc.).
+ */
+export function getRequestProductConfig(req: FastifyRequest): ProductDoc {
+  const id = getRequestProductId(req);
+  return getProduct(id)!;
+}