Show P1/A1 cameras as a refreshing picture

SnapshotView fetches one frame at a time through the backend, pauses
while the tab is hidden and backs off after failures. PrinterCard picks
it when /stream answers kind "snapshot".

apps/web/composables/usePrinterStream.ts

@@ -1,7 +1,13 @@
 /**
  * Where a printer's camera can be watched, from `GET /printers/:id/stream`.
  */
-export interface PrinterStream {
+export type PrinterStream =
+  | WebrtcStream
+  // A P1/A1: no stream to negotiate, a picture to fetch again and again.
+  | { kind: "snapshot"; snapshotPath: string; intervalMs: number };
+
+export interface WebrtcStream {
+  kind?: "webrtc";
   /** The go2rtc stream name to negotiate for. */
   cameraId: string;
   /**