Move version display to GET /api/health/version in HealthController

Uses GetApplicationVersion(Assembly.GetExecutingAssembly()) — the same
timestamp-based version already logged at startup and baked into the
assembly via the csproj <Version> property. Removes the minimal-API
/version endpoint from web/Program.cs and reverts the web Dockerfile
APP_VERSION build-arg (no longer needed).

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

web/Program.cs

@@ -14,9 +14,6 @@ var app = builder.Build();
 
 app.UseMiddleware<SiteModeMiddleware>();
 
-app.MapGet("/version", (IConfiguration config) =>
-    Results.Json(new { version = config["APP_VERSION"] ?? "unknown" }));
-
 // Static site
 app.UseDefaultFiles();
 app.UseStaticFiles();