Implement product multi-buys and variants system

Major restructuring of product variations:
- Renamed ProductVariation to ProductMultiBuy for quantity-based pricing (e.g., "3 for £25")
- Added new ProductVariant model for string-based options (colors, flavors)
- Complete separation of multi-buy pricing from variant selection

Features implemented:
- Multi-buy deals with automatic price-per-unit calculation
- Product variants for colors/flavors/sizes with stock tracking
- TeleBot checkout supports both multi-buys and variant selection
- Shopping cart correctly calculates multi-buy bundle prices
- Order system tracks selected variants and multi-buy choices
- Real-time bot activity monitoring with SignalR
- Public bot directory page with QR codes for Telegram launch
- Admin dashboard shows multi-buy and variant metrics

Technical changes:
- Updated all DTOs, services, and controllers
- Fixed cart total calculation for multi-buy bundles
- Comprehensive test coverage for new functionality
- All existing tests passing with new features

Database changes:
- Migrated ProductVariations to ProductMultiBuys
- Added ProductVariants table
- Updated OrderItems to track variants

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

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

LittleShop/Controllers/ProductVariantsController.cs

@@ -0,0 +1,81 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using LittleShop.DTOs;
+using LittleShop.Services;
+
+namespace LittleShop.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize(AuthenticationSchemes = "Bearer")]
+public class ProductVariantsController : ControllerBase
+{
+    private readonly IProductService _productService;
+
+    public ProductVariantsController(IProductService productService)
+    {
+        _productService = productService;
+    }
+
+    [HttpGet("product/{productId}")]
+    public async Task<ActionResult<IEnumerable<ProductVariantDto>>> GetProductVariants(Guid productId)
+    {
+        var variants = await _productService.GetProductVariantsAsync(productId);
+        return Ok(variants);
+    }
+
+    [HttpGet("{id}")]
+    public async Task<ActionResult<ProductVariantDto>> GetProductVariant(Guid id)
+    {
+        var variant = await _productService.GetProductVariantByIdAsync(id);
+        if (variant == null)
+            return NotFound();
+
+        return Ok(variant);
+    }
+
+    [HttpPost]
+    [Authorize(Roles = "Admin")]
+    public async Task<ActionResult<ProductVariantDto>> CreateProductVariant(CreateProductVariantDto createVariantDto)
+    {
+        try
+        {
+            var variant = await _productService.CreateProductVariantAsync(createVariantDto);
+            return CreatedAtAction(nameof(GetProductVariant), new { id = variant.Id }, variant);
+        }
+        catch (ArgumentException ex)
+        {
+            return BadRequest(ex.Message);
+        }
+    }
+
+    [HttpPut("{id}")]
+    [Authorize(Roles = "Admin")]
+    public async Task<IActionResult> UpdateProductVariant(Guid id, UpdateProductVariantDto updateVariantDto)
+    {
+        try
+        {
+            await _productService.UpdateProductVariantAsync(id, updateVariantDto);
+            return NoContent();
+        }
+        catch (ArgumentException ex)
+        {
+            return BadRequest(ex.Message);
+        }
+    }
+
+    [HttpDelete("{id}")]
+    [Authorize(Roles = "Admin")]
+    public async Task<IActionResult> DeleteProductVariant(Guid id)
+    {
+        try
+        {
+            await _productService.DeleteProductVariantAsync(id);
+            return NoContent();
+        }
+        catch (ArgumentException ex)
+        {
+            return BadRequest(ex.Message);
+        }
+    }
+}