Relay P1/A1 camera frames to viewers

GET /printers/:id/snapshot asks the farm's gateway for one JPEG frame
(camera_snapshot_request) and returns it no-store. Concurrent viewers
share one read and a frame under 800 ms old is reused, because the
printer's camera serves one client. /stream answers kind "snapshot" for
these printers; printers.camera_snapshot is set from telemetry.

scripts/e2e-camera-snapshot.sh reads real frames from the P1P through
the real proxy and backend.

src/db/schema/printers.ts

@@ -1,4 +1,4 @@
-import { pgTable, uuid, text, timestamp, jsonb, pgEnum, real, integer, uniqueIndex } from "drizzle-orm/pg-core";
+import { pgTable, uuid, text, timestamp, jsonb, pgEnum, real, integer, boolean, uniqueIndex } from "drizzle-orm/pg-core";
 import { farms } from "./farms";
 
 export const printerVendorEnum = pgEnum("printer_vendor", ["bambu", "prusa", "klipper"]);
@@ -70,6 +70,11 @@ export const printers = pgTable(
      * restream it.
      */
     cameraStream: text("camera_stream"),
+    /**
+     * The camera can only be read as single JPEG frames (a P1/A1), which the
+     * gateway serves on request. Mutually exclusive with `cameraStream`.
+     */
+    cameraSnapshot: boolean("camera_snapshot").notNull().default(false),
     /**
      * The printer's own state string, kept alongside the narrowed
      * `status` enum. Worth storing both: the enum is what the UI filters