Add R2 image storage, upload endpoint, and CDN support

- Backend: R2StorageService, upload controller (POST /api/upload)
- Frontend: CDN url helper, NEXT_PUBLIC_CDN_URL build arg
- Deploy: pass R2 secrets from Woodpecker CI to containers via .env
- Docs: update CLAUDE.md with CDN and upload conventions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/GBSite.Api/GBSite.Api.csproj

@@ -4,9 +4,11 @@
     <TargetFramework>net9.0</TargetFramework>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
+    <UserSecretsId>805cad54-8a19-4713-b893-a1ec63696146</UserSecretsId>
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="AWSSDK.S3" Version="3.7.*" />
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.4" />
     <PackageReference Include="Npgsql" Version="10.0.1" />
   </ItemGroup>