fix(lora-worker): allowlist /storetank/arbo/models as the canonical base-model root

The first real arbo train 422'd: SDXL checkpoints live at
/storetank/arbo/models/checkpoints/ (the 2026-06-13 move to the 1.8TB
/storetank volume), which wasn't in ALLOWED_MODEL_ROOTS — the old roots
predated the move (/worktank/models is gone, /worktank/comfyui host path
is empty; ComfyUI mounts /storetank/arbo/models -> /basedir/models inside
its container). Allowlist /storetank/arbo/models (llmuser-readable,
world-readable tree), drop the two stale roots. Regression test added (15 green).

services/lora-training-worker/tests/test_invocation.py

@@ -100,6 +100,15 @@ def test_base_model_outside_allowed_roots_rejected():
         build_command(_req(base_model_path="/home/someone/evil.safetensors"))
 
 
+def test_storetank_checkpoint_allowed():
+    # the canonical SDXL store (2026-06-13 move) — arbo dispatches base_model_path from here
+    argv, _, _ = build_command(
+        _req(base_model_path="/storetank/arbo/models/checkpoints/albedobaseXL_v31Large.safetensors")
+    )
+    assert _flag_value(argv, "--pretrained_model_name_or_path") == \
+        "/storetank/arbo/models/checkpoints/albedobaseXL_v31Large.safetensors"
+
+
 def test_unsafe_output_name_rejected():
     with pytest.raises(InvalidTrainRequest, match="output_name"):
         build_command(_req(output_name="a; rm -rf /"))