🔒 SECURITY: Emergency fixes and hardening

EMERGENCY FIXES:
✅ DELETE MockSilverPayService.cs - removed fake payment system
✅ REMOVE mock service registration - no fake payments possible
✅ GENERATE new JWT secret - replaced hardcoded key
✅ FIX HttpClient disposal - proper resource management

SECURITY HARDENING:
✅ ADD production guards - prevent mock services in production
✅ CREATE environment configs - separate dev/prod settings
✅ ADD config validation - fail fast on misconfiguration

IMPACT:
- Mock payment system completely eliminated
- JWT authentication now uses secure keys
- Production deployment now validated on startup
- Resource leaks fixed in TeleBot currency API

🤖 Generated with [Claude Code](https://claude.ai/code)

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

LittleShop/Migrations/20250922040637_AddSystemSettingsTable.cs

@@ -0,0 +1,43 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace LittleShop.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddSystemSettingsTable : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "SystemSettings",
+                columns: table => new
+                {
+                    Key = table.Column<string>(type: "TEXT", nullable: false),
+                    Value = table.Column<string>(type: "TEXT", nullable: false),
+                    Description = table.Column<string>(type: "TEXT", nullable: true),
+                    CreatedAt = table.Column<DateTime>(type: "TEXT", nullable: false),
+                    UpdatedAt = table.Column<DateTime>(type: "TEXT", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_SystemSettings", x => x.Key);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_SystemSettings_Key",
+                table: "SystemSettings",
+                column: "Key",
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "SystemSettings");
+        }
+    }
+}