Make API auth mandatory, manage tokens via web UI

BREAKING CHANGE: API authentication is now always required. The
PINCHFLAT_API_TOKEN env var is no longer used. Instead, tokens are
stored in the database and managed via Settings → API Access.

Changes:
- Add api_token column to settings table (migration)
- ApiAuthPlug reads from DB; returns 401 if no token configured
- Add API Access section to Settings page with generate/regenerate/revoke
- Add POST /settings/generate_api_token and /settings/revoke_api_token
- Remove api_token from config.exs and runtime.exs
- Update all API controller tests to set auth token in setup
- Update auth plug tests for mandatory authentication
- 1008 tests pass, zero warnings

lib/pinchflat_web/controllers/settings/setting_controller.ex

@@ -28,6 +28,36 @@ defmodule PinchflatWeb.Settings.SettingController do
     render(conn, "app_info.html")
   end
 
+  def generate_api_token(conn, _params) do
+    token = :crypto.strong_rand_bytes(32) |> Base.url_encode64(padding: false)
+
+    case Settings.set(api_token: token) do
+      {:ok, _} ->
+        conn
+        |> put_flash(:info, "API token generated successfully.")
+        |> redirect(to: ~p"/settings")
+
+      {:error, _} ->
+        conn
+        |> put_flash(:error, "Failed to generate API token.")
+        |> redirect(to: ~p"/settings")
+    end
+  end
+
+  def revoke_api_token(conn, _params) do
+    case Settings.set(api_token: nil) do
+      {:ok, _} ->
+        conn
+        |> put_flash(:info, "API token revoked.")
+        |> redirect(to: ~p"/settings")
+
+      {:error, _} ->
+        conn
+        |> put_flash(:error, "Failed to revoke API token.")
+        |> redirect(to: ~p"/settings")
+    end
+  end
+
   def download_logs(conn, _params) do
     log_path = Application.get_env(:pinchflat, :log_path)