feat(web): SPA cutover — root route + legacy redirects (#91)

Make the ChicoryTV SPA the default UI: GET/HEAD requests to / and to
legacy Blazor routes with SPA equivalents (channels, schedules,
playouts, libraries, all settings pages, channel add) now 302 to their
/app counterparts, preserving query strings and the ETV_BASE_URL path
base. 302 not 301: the map will grow as parity lands and permanent-
redirect caching would make rollback painful.

Blazor-only functionality (collections, media browse/search, trakt,
filler presets, watermarks, ffmpeg profiles, blocks/decos/templates,
playout detail editors, logs, troubleshooting, channel edit) keeps
serving Blazor; the Blazor home stays reachable at /system/health.
Parity gaps are tracked in #140-#146; Blazor removal is phase (b).

Also adds an /app smoke assertion to the docker build workflow.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV/LegacyUiRedirects.cs

@@ -0,0 +1,74 @@
+using Microsoft.AspNetCore.Http;
+
+namespace ErsatzTV;
+
+// Phase (a) of the Blazor -> ChicoryTV SPA cutover (ersatztv#91).
+//
+// The React SPA (served under /app) is now the default UI: the legacy Blazor
+// routes below 302-redirect to their SPA equivalents. Only routes that already
+// have SPA parity are listed here. Blazor pages WITHOUT a SPA equivalent are
+// deliberately left reachable (no redirect) so their functionality stays
+// available while the SPA catches up:
+//   /system/health (Blazor home escape hatch; Index.razor also lives here),
+//   /channels/{id} edit, /channels/numbers, /media/* (collections etc.),
+//   /ffmpeg, /watermarks, /blocks, /decos, /templates, /deco-templates,
+//   schedule/playout detail editors, /system/logs, /system/troubleshooting.
+//
+// Phase (b) removes the redirected Blazor pages entirely, but that is GATED on
+// full SPA parity for every route in this map. Until then this map is the
+// single source of truth for "what has migrated" and is expected to grow.
+public static class LegacyUiRedirects
+{
+    // Blazor route -> SPA route. EXACT paths only (no prefix matching); a single
+    // trailing slash on the request is normalized away before lookup.
+    public static readonly IReadOnlyDictionary<string, string> Map =
+        new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+        {
+            ["/"] = "/app",
+            ["/channels"] = "/app/channels",
+            ["/channels/add"] = "/app/new-channel",
+            ["/schedules"] = "/app/schedules",
+            ["/playouts"] = "/app/playouts",
+            ["/media/libraries"] = "/app/libraries",
+            ["/settings/ffmpeg"] = "/app/settings/streaming",
+            ["/settings/hdhr"] = "/app/settings/system",
+            ["/settings/logging"] = "/app/settings/logging",
+            ["/settings/playout"] = "/app/settings/playout",
+            ["/settings/scanner"] = "/app/settings/scanner",
+            ["/settings/ui"] = "/app/settings/general",
+            ["/settings/xmltv"] = "/app/settings/xmltv"
+        };
+
+    public static bool TryGetRedirect(PathString path, out string target)
+    {
+        target = string.Empty;
+
+        string value = path.Value;
+        if (string.IsNullOrEmpty(value))
+        {
+            return false;
+        }
+
+        // Normalize a single trailing slash so "/channels/" matches "/channels"
+        // (but keep root "/" intact). Guard against a path of all slashes (e.g.
+        // "//") collapsing down to "/" and falsely matching the root entry.
+        if (value.Length > 1 && value.EndsWith('/'))
+        {
+            string trimmed = value[..^1];
+            if (trimmed == "/")
+            {
+                return false;
+            }
+
+            value = trimmed;
+        }
+
+        if (Map.TryGetValue(value, out string mapped))
+        {
+            target = mapped;
+            return true;
+        }
+
+        return false;
+    }
+}

ErsatzTV/Startup.cs

@@ -726,6 +726,28 @@ public class Startup
             ctx => !IsIptvPath(ctx.Request.Path) && !IsSpaPath(ctx.Request.Path),
             blazor =>
             {
+                // ersatztv#91 phase (a): make the ChicoryTV SPA the default UI by
+                // redirecting migrated legacy Blazor routes to their /app equivalents.
+                // 302 (not 301): this map grows as pages migrate, and permanent-redirect
+                // browser caching would make rollback painful. UsePathBase (ETV_BASE_URL)
+                // only rewrites the request side (Request.Path/PathBase); it never touches
+                // redirect Location headers, so the PathBase prefix must be re-applied here.
+                blazor.Use(async (context, next) =>
+                {
+                    if (HttpMethods.IsGet(context.Request.Method) ||
+                        HttpMethods.IsHead(context.Request.Method))
+                    {
+                        if (LegacyUiRedirects.TryGetRedirect(context.Request.Path, out string target))
+                        {
+                            context.Response.Redirect(
+                                context.Request.PathBase + target + context.Request.QueryString);
+                            return;
+                        }
+                    }
+
+                    await next(context);
+                });
+
                 blazor.UseRouting();
 
                 if (OidcHelper.IsEnabled)