chore: initial project structure and root configuration

Adds root-level config files: solution (.slnx), NuGet, global.json,
Docker Compose files for all services (API, dashboard, website, finder,
admin), environment example, and developer documentation.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nuget.config

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+  </packageSources>
+  <config>
+    <!-- Helps flaky TLS / filtered networks during docker build restore -->
+    <add key="http_retry_count" value="8" />
+  </config>
+</configuration>