add no-JS fallback for provider forms

Progressive enhancement: provider form now works without JavaScript.
Forms POST to ProvidersController (create/update), which handles
validation and redirects with flash messages.

With JS: LiveView phx-submit handles save, navigates with flash.
Without JS: Form POSTs to controller, redirects with flash.

Completes Task 3 of notification overhaul (admin forms migration).

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

lib/berrypod_web/controllers/providers_controller.ex

@@ -0,0 +1,54 @@
+defmodule BerrypodWeb.ProvidersController do
+  @moduledoc """
+  No-JS fallback for provider connection forms.
+
+  With JS enabled, the LiveView handles everything. Without JS,
+  the form POSTs here and we redirect back to the settings page.
+  """
+  use BerrypodWeb, :controller
+
+  alias Berrypod.{KeyValidation, Products}
+  alias Berrypod.Providers.Provider
+
+  def create(conn, %{"provider_connection" => params}) do
+    api_key = params["api_key"]
+    provider_type = params["provider_type"]
+    provider = Provider.get(provider_type)
+
+    case KeyValidation.validate_provider_key(api_key, provider_type) do
+      {:error, message} ->
+        conn
+        |> put_flash(:error, message)
+        |> redirect(to: ~p"/admin/providers/new?type=#{provider_type}")
+
+      {:ok, api_key} ->
+        case Products.connect_provider(api_key, provider_type) do
+          {:ok, _connection} ->
+            conn
+            |> put_flash(:info, "Connected to #{provider.name}!")
+            |> redirect(to: ~p"/admin/settings")
+
+          {:error, _reason} ->
+            conn
+            |> put_flash(:error, "Could not connect. Check your API key and try again")
+            |> redirect(to: ~p"/admin/providers/new?type=#{provider_type}")
+        end
+    end
+  end
+
+  def update(conn, %{"id" => id, "provider_connection" => params}) do
+    connection = Products.get_provider_connection!(id)
+
+    case Products.update_provider_connection(connection, params) do
+      {:ok, _connection} ->
+        conn
+        |> put_flash(:info, "Settings saved")
+        |> redirect(to: ~p"/admin/settings")
+
+      {:error, _changeset} ->
+        conn
+        |> put_flash(:error, "Could not save settings")
+        |> redirect(to: ~p"/admin/providers/#{id}/edit")
+    end
+  end
+end