feat(camera): optional snapshot URL override for external cameras (#1177)

go2rtc and several IP cameras still emit a warm-up / black frame on every
  fresh MJPEG connection — even with the v0.2.4b2 warm-up-skip fix it
  slipped through intermittently for @nkm8's setup. His own bisect named
  the clean solution: go2rtc exposes /api/frame.jpeg as a dedicated
  single-frame endpoint that never returns the encoder's stale keyframe.

  Adds an optional external_camera_snapshot_url column on printers. When
  set, every single-frame capture path (snapshot endpoint, [SNAPSHOT]
  notification thumbnails, [PHOTO-BG] finish photo, layer timelapse,
  Obico ML, plate-detect / calibrate-plate) routes through _capture_snapshot
  on the override URL via plain HTTP GET, bypassing the warm-up dance.

  Live view stays on the configured stream URL — only single-frame
  captures use the override. Override is camera-type-agnostic. SSRF guard
  applies (existing _sanitize_camera_url allowlist). Empty string treated
  as unset.

  Settings UI: new "Snapshot URL (optional)" input + Test button under
  External Cameras, hidden for camera_type=snapshot since the live URL is
  already a single-frame source. en + de fully translated; 6 other locales
  seeded with English copy.

  5 backend tests pin the routing contract; 3 frontend tests pin the
  input + debounced PATCH. Documented in
  bambuddy-wiki/docs/features/camera.md with the go2rtc example.

backend/app/api/routes/camera.py

@@ -792,7 +792,12 @@ async def camera_snapshot(
     if printer.external_camera_enabled and printer.external_camera_url:
         from backend.app.services.external_camera import capture_frame
 
-        frame_data = await capture_frame(printer.external_camera_url, printer.external_camera_type, timeout=15)
+        frame_data = await capture_frame(
+            printer.external_camera_url,
+            printer.external_camera_type,
+            timeout=15,
+            snapshot_url=printer.external_camera_snapshot_url,
+        )
         if not frame_data:
             raise HTTPException(
                 status_code=503,
@@ -1040,6 +1045,7 @@ async def check_plate_empty(
         external_camera_type=printer.external_camera_type if printer.external_camera_enabled else None,
         use_external=use_external,
         roi=roi,
+        external_camera_snapshot_url=printer.external_camera_snapshot_url if printer.external_camera_enabled else None,
     )
 
     # Get reference count for the response
@@ -1124,6 +1130,7 @@ async def calibrate_plate_detection(
         external_camera_url=printer.external_camera_url if printer.external_camera_enabled else None,
         external_camera_type=printer.external_camera_type if printer.external_camera_enabled else None,
         use_external=use_external,
+        external_camera_snapshot_url=printer.external_camera_snapshot_url if printer.external_camera_enabled else None,
     )
 
     if light_warning and success: