Cache-busting so deploys are visible immediately

The site sits behind a CDN and shipped static assets with no Cache-Control
and no versioning, so browsers/CDN kept serving stale css/js after a deploy
(the "design didn't change" symptom).

- HTML responses now send Cache-Control: no-cache, no-store, must-revalidate
  so the page itself is always revalidated.
- Static assets get a long cache and are fingerprinted via asp-append-version
  (/css/site.css?v=<contenthash>), so they bust automatically on every change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Program.cs

@@ -50,7 +50,29 @@ using (var scope = app.Services.CreateScope())
 }
 
 app.UseStatusCodePagesWithReExecute("/Error/{0}");
-app.UseStaticFiles();
+
+// HTML pages must never be cached by the browser or CDN, so a new deploy is
+// visible immediately. Static assets are fingerprinted via asp-append-version
+// (?v=hash), so they can be cached aggressively and bust automatically.
+app.Use(async (context, next) =>
+{
+    context.Response.OnStarting(() =>
+    {
+        if (context.Response.ContentType?.Contains("text/html", StringComparison.OrdinalIgnoreCase) == true)
+        {
+            context.Response.Headers.CacheControl = "no-cache, no-store, must-revalidate";
+            context.Response.Headers.Pragma = "no-cache";
+        }
+        return Task.CompletedTask;
+    });
+    await next();
+});
+
+app.UseStaticFiles(new StaticFileOptions
+{
+    OnPrepareResponse = ctx =>
+        ctx.Context.Response.Headers.CacheControl = "public, max-age=31536000"
+});
 
 // Serve uploaded files from /data/uploads under /uploads/*
 var uploadsPath = Path.Combine(dataDir, "uploads");