refactor: extract import_garmin_gear() + add backfill script

Move gear backfill logic from the route handler into
import_garmin_gear(data_dir, user_dir) in garmin_sync.py so it can be
called both from the API and from the CLI script.

scripts/backfill_garmin_gear.py finds all users with Garmin credentials
and runs the backfill for each, printing a per-user summary.

scripts/backfill_garmin_gear.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+"""Backfill Garmin gear for all users who have stored Garmin credentials.
+
+Usage (on VPS):
+    cd /opt/bincio
+    uv run python3 scripts/backfill_garmin_gear.py --data-dir /var/bincio/data
+
+    # Limit to specific users:
+    uv run python3 scripts/backfill_garmin_gear.py --data-dir /var/bincio/data --users plagzo12
+"""
+from __future__ import annotations
+
+import argparse
+import sys
+from pathlib import Path
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description="Backfill Garmin gear for all users")
+    parser.add_argument("--data-dir", required=True, type=Path, help="Root data directory")
+    parser.add_argument("--users", nargs="*", help="Limit to these user handles (default: all)")
+    args = parser.parse_args()
+
+    data_dir: Path = args.data_dir.resolve()
+    if not data_dir.is_dir():
+        sys.exit(f"data-dir not found: {data_dir}")
+
+    from bincio.extract.garmin_api import GarminError, has_credentials
+    from bincio.extract.garmin_sync import import_garmin_gear
+
+    candidates = (
+        [data_dir / h for h in args.users]
+        if args.users
+        else sorted(p for p in data_dir.iterdir() if p.is_dir())
+    )
+
+    garmin_users = [p for p in candidates if has_credentials(p)]
+    if not garmin_users:
+        print("No users with Garmin credentials found.")
+        return
+
+    print(f"Found {len(garmin_users)} Garmin user(s): {[p.name for p in garmin_users]}\n")
+
+    for user_dir in garmin_users:
+        handle = user_dir.name
+        print(f"[{handle}] importing gear...", flush=True)
+        try:
+            result = import_garmin_gear(data_dir, user_dir)
+            print(
+                f"[{handle}] done — "
+                f"gear_added={result['gear_added']}, "
+                f"activities_updated={result['activities_updated']}"
+            )
+        except GarminError as exc:
+            print(f"[{handle}] Garmin error: {exc}")
+        except Exception as exc:
+            print(f"[{handle}] unexpected error: {type(exc).__name__}: {exc}")
+
+    print("\nAll done. Run merge_all or trigger a rebuild to refresh the index shards.")
+
+
+if __name__ == "__main__":
+    main()