Fix: capture timelapse baseline on expected-archive on_print_start branch (#1403 follow-up)

The snapshot-diff strategy in _scan_for_timelapse_with_retries needs
  _timelapse_baselines[printer_id] populated at print start so the
  completion-time scan can find the new MP4 by set-difference (mtime is
  unreliable — LAN-only printers don't sync NTP).

  The baseline-capture call was only in on_print_start's new-archive branch.
  Queue / VP-dispatched / reprinted jobs take the expected-archive branch
  which returns earlier, so the dict stayed empty and the completion-time
  scan fell into the "take baseline now" fallback that snapshots after the
  new file has already landed — no diff ever matches.

  Extract the snapshot into _capture_timelapse_baseline_at_start and call
  it from both branches.

backend/app/main.py

@@ -2121,6 +2121,14 @@ async def on_print_start(printer_id: int, data: dict):
                 except Exception as e:
                     logger.warning("[SPOOLMAN] Failed to store tracking data: %s", e)
 
+                # Capture timelapse file baseline for snapshot-diff on completion
+                # (mirrors the new-archive branch). Queue / VP-dispatched prints
+                # hit this branch — without the baseline the completion-time scan
+                # falls into its "take baseline now" fallback, which snapshots
+                # AFTER the new MP4 already exists and never matches a diff
+                # (#1403 follow-up — see pwostran's 2026-05-18 support bundle).
+                await _capture_timelapse_baseline_at_start(printer, printer_id, logger)
+
             return  # Skip creating a new archive
 
         # Check if there's already a "printing" archive for this printer/file
@@ -2725,16 +2733,7 @@ async def on_print_start(printer_id: int, data: dict):
                     logger.warning("[SPOOLMAN] Failed to store tracking data: %s", e)
 
                 # Capture timelapse file baseline for snapshot-diff on completion
-                try:
-                    baseline_files, _ = await _list_timelapse_videos(printer)
-                    _timelapse_baselines[printer_id] = {f.get("name", "") for f in baseline_files}
-                    logger.info(
-                        "[TIMELAPSE] Baseline at print start: %s video files for printer %s",
-                        len(_timelapse_baselines[printer_id]),
-                        printer_id,
-                    )
-                except Exception as e:
-                    logger.warning("[TIMELAPSE] Failed to capture baseline at print start: %s", e)
+                await _capture_timelapse_baseline_at_start(printer, printer_id, logger)
         finally:
             # Keep temp_path around until print completes so the cover endpoint
             # can reuse it (#972). Cache eviction in on_print_complete deletes
@@ -2779,6 +2778,32 @@ async def _list_timelapse_videos(printer) -> tuple[list[dict], str | None]:
     return [], None
 
 
+async def _capture_timelapse_baseline_at_start(printer, printer_id: int, logger: logging.Logger) -> None:
+    """Snapshot the printer's timelapse directory at print start so the
+    completion-time scan can pick the new file by set-difference.
+
+    Must be called from every on_print_start path that proceeds to a real
+    print — both the new-archive branch and the expected-archive branch (which
+    queue / VP-dispatched prints take). Without a baseline,
+    _scan_for_timelapse_with_retries falls into its "take baseline now"
+    fallback that runs AFTER the new MP4 has already landed on the SD card,
+    so the new file ends up in the "baseline" set and no diff ever matches.
+
+    Bambu printers in LAN-only mode don't sync NTP, so mtime ordering is
+    unreliable — the snapshot-diff approach sidesteps that entirely.
+    """
+    try:
+        baseline_files, _ = await _list_timelapse_videos(printer)
+        _timelapse_baselines[printer_id] = {f.get("name", "") for f in baseline_files}
+        logger.info(
+            "[TIMELAPSE] Baseline at print start: %s video files for printer %s",
+            len(_timelapse_baselines[printer_id]),
+            printer_id,
+        )
+    except Exception as e:
+        logger.warning("[TIMELAPSE] Failed to capture baseline at print start: %s", e)
+
+
 async def _scan_for_timelapse_with_retries(archive_id: int, baseline_names: set[str] | None = None):
     """
     Scan for timelapse with retries using a snapshot-diff approach.