fix(training): disable DataLoader workers in subprocess training

DataLoader worker subprocesses crash inside Docker due to multiprocessing
fork restrictions. Pass --workers 0 to both ketos train and ketos segtrain
so data loading runs in the main process.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/main.py

@@ -366,7 +366,8 @@ async def train_model(
             os.makedirs(checkpoint_dir, exist_ok=True)
 
             cmd = [
-                "ketos", "train",
+                "ketos", "--workers", "0",
+                "train",
                 "-f", "path",
                 "-o", checkpoint_dir,
                 "-q", "fixed",
@@ -456,7 +457,8 @@ async def segtrain_model(
             os.makedirs(checkpoint_dir, exist_ok=True)
 
             cmd = [
-                "ketos", "segtrain",
+                "ketos", "--workers", "0",
+                "segtrain",
                 "-o", checkpoint_dir,
                 "-q", "fixed",
                 "-N", "50",