Wire product→merchant-profile write path

Multi-profile resolution shipped in :52 but nothing wrote
products.merchant_profile_id, so it was non-functional end to end.

Add merchant_profile_id to the Product model + all four product
SELECTs, a set_product_merchant_profile writer (validates the target
profile exists, returning 404 instead of a raw FK-violation 500), and
thread an optional field through CreateProductReq (post-write) and
UpdateProductReq (double-Option; Some(None) clears to default). The
admin SPA product form shows a profile picker only when >1 profile
exists. Mirrors the entitlements-catalog post-write pattern.

Tests: repo round-trip (attach/resolve/clear/bad-id) + HTTP handler
arms. api suite 54→56, full suite green.

licensing-service/src/models.rs

@@ -34,6 +34,12 @@ pub struct Product {
     /// behavior); operators can opt-in by adding rows.
     #[serde(default)]
     pub entitlements_catalog: Option<Vec<EntitlementDef>>,
+    /// Merchant profile this product belongs to (migration 0020). None
+    /// resolves to the default profile (back-compat for rows created
+    /// before the operator ran more than one profile). Set via the admin
+    /// product form when >1 profile exists.
+    #[serde(default)]
+    pub merchant_profile_id: Option<String>,
     pub created_at: String,
     pub updated_at: String,
 }