Extract store helpers into storage_helpers.py

auto_out_path, load_store, save_store moved out of data_model.py into
their own module. data_model.py now contains only value helpers and
domain extractors.

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

storage_helpers.py

@@ -0,0 +1,35 @@
+import json
+import logging
+from datetime import datetime, timezone
+from pathlib import Path
+
+log = logging.getLogger(__name__)
+
+
+def auto_out_path(logs_dir: Path, vin: str) -> Path:
+    stamp = datetime.now(timezone.utc).strftime("%Y_%m_%d_%H_%M_%S")
+    return logs_dir / f"{stamp}_{vin}.json"
+
+
+def load_store(path: Path, vin: str, interval: int) -> dict:
+    if path.exists():
+        try:
+            data = json.loads(path.read_text())
+            if isinstance(data, dict) and "records" in data:
+                return data
+        except json.JSONDecodeError:
+            log.warning("Output file is corrupt — starting fresh")
+    return {
+        "meta": {
+            "vin": vin,
+            "created_at": datetime.now(timezone.utc).isoformat(),
+            "interval_seconds": interval,
+        },
+        "records": [],
+    }
+
+
+def save_store(path: Path, store: dict, max_records: int | None) -> None:
+    if max_records and len(store["records"]) > max_records:
+        store["records"] = store["records"][-max_records:]
+    path.write_text(json.dumps(store, indent=2, default=str))