Strava sync: skip import when a FIT-file upload already covers the same start time

Before importing each Strava activity, build a set of existing timestamp
prefixes (YYYY-MM-DDTHHMMSSZ) from the activities directory. If the incoming
Strava activity matches an existing prefix, record its Strava ID as done and
skip — preventing duplicate entries when a FIT file and a Strava sync both
cover the same ride.

Also reports skipped-existing count in the summary line.

bincio/import_/strava.py

@@ -343,9 +343,24 @@ def sync(
     owner = index_data.get("owner", {})
     summaries: dict[str, dict] = {s["id"]: s for s in index_data.get("activities", [])}
 
+    # ── build timestamp-prefix index of existing activities ──────────────────
+    # Maps "YYYY-MM-DDTHHMMSSZ" → first matching activity filename (stem).
+    # Used to detect when a FIT-file upload already covers a Strava activity.
+    acts_dir = output_dir / "activities"
+    existing_ts: set[str] = set()
+    if acts_dir.is_dir():
+        for p in acts_dir.iterdir():
+            if p.suffix == ".json" and not p.name.endswith(".timeseries.json"):
+                stem = p.stem
+                # ID format: YYYY-MM-DDTHHMMSSZ[-optional-slug]
+                z_pos = stem.find("Z")
+                if z_pos != -1:
+                    existing_ts.add(stem[: z_pos + 1])
+
     # ── import loop ───────────────────────────────────────────────────────────
     errors: list[tuple[str, str]] = []
     imported = 0
+    skipped_existing = 0
 
     with Progress(
         TextColumn("[progress.description]{task.description}"),
@@ -362,11 +377,20 @@ def sync(
             try:
                 streams  = client.get_streams(act["id"])
                 parsed   = _strava_to_parsed(act, streams)
+
+                # Skip if any activity already exists for the same start time
+                ts_part = parsed.started_at.astimezone(timezone.utc).strftime("%Y-%m-%dT%H%M%SZ")
+                if ts_part in existing_ts:
+                    imported_ids.add(strava_id)
+                    skipped_existing += 1
+                    continue
+
                 metrics  = compute(parsed)
                 metrics  = _patch_from_summary(metrics, act)
                 act_id   = make_activity_id(parsed)
                 write_activity(parsed, metrics, output_dir, privacy="public")
                 summaries[act_id] = build_summary(parsed, metrics, act_id, "public")
+                existing_ts.add(ts_part)
                 imported_ids.add(strava_id)
                 imported += 1
             except Exception as exc:
@@ -384,9 +408,11 @@ def sync(
         from bincio.render.merge import merge_all
         merge_all(output_dir)
 
+    skipped_msg = f", skipped [bold]{skipped_existing}[/bold] already covered by local uploads" if skipped_existing else ""
     console.print(
         f"\n[green]Done.[/green] "
-        f"Imported [bold]{imported}[/bold] activities, "
+        f"Imported [bold]{imported}[/bold] activities"
+        f"{skipped_msg}, "
         f"errors [bold]{len(errors)}[/bold]."
     )
     if errors: