feat: V2 microservices stack — backend services, gateway, JWT auth

Add full V2 architecture: identity, content, studio (.NET 10) and file,
render, notification, gateway (Go) services with vendored deps, plus DB
migrations, event/API contracts, and an init-db script.

Wire the Next.js frontend to the gateway: server-side JWT auth routes
(login/register/refresh/logout/me), gateway fetch helper, and session/
cookie/jwt helpers under src/lib.

Containerize the stack via docker-compose.v2.yml and per-service
Dockerfiles. Base images resolve through a Nexus mirror (Docker Hub) and
MCR directly; npm/NuGet pull from Nexus groups. Self-host fonts via
next/font/local to avoid Google Fonts (geo-blocked).

Add CI workflow and ignore .env.v2, *.stackdump, and .NET bin/obj.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

services/identity/FlatRender.IdentitySvc/Models/Requests/BillingRequests.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace FlatRender.IdentitySvc.Models.Requests;
+
+public record PurchasePlanRequest(
+    [Required] Guid PlanId,
+    string? Gateway,
+    string? DiscountCode
+);
+
+public record ValidateDiscountRequest([Required] string Code, Guid? PlanId);
+
+public record CreateDiscountRequest(
+    [Required] string Name,
+    [Required] string Code,
+    [Required] string Kind,
+    [Required] decimal Value,
+    Guid? OwnerUserId,
+    decimal OwnerProfitPercentage = 0,
+    int? MaxUseCount = null,
+    Guid[]? AppliesToPlanIds = null,
+    DateTime? StartsAt = null,
+    DateTime? ExpiresAt = null
+);
+
+public record IssueRefundRequest(
+    [Required] Guid PaymentId,
+    long? AmountMinor,
+    [Required] string Reason,
+    string RefundTo = "Balance"
+);