Fix Garmin full export import: UDSFile health data and nested zip FIT files

Garmin Connect exports use UDSFile_*.json (not DailyMetrics) for daily
wellness summaries, and pack activity FIT files inside nested sub-zips
under DI-Connect-Uploaded-Files/ rather than at the top level.

- process_garmin_health_zip: match UDSFile_*.json instead of DailyMetrics,
  handle list-of-records format, extract stress from allDayStress.aggregatorList,
  convert floorsAscendedInMeters to floor count
- upload_garmin_export: recurse into nested .zip files to find and queue
  individual activity FIT files

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

backend/app/api/upload.py

@@ -75,6 +75,22 @@ async def upload_garmin_export(
                 fit_path = extract_dir / name
                 task = process_activity_file.delay(str(fit_path), current_user.id, "fit")
                 task_ids.append(task.id)
+            elif lower.endswith(".zip"):
+                # Garmin exports nest activity FIT files inside sub-zips
+                # (e.g. DI-Connect-Uploaded-Files/UploadedFiles_*_Part*.zip)
+                nested_zip_path = extract_dir / name
+                nested_extract = nested_zip_path.parent / nested_zip_path.stem
+                nested_extract.mkdir(exist_ok=True)
+                try:
+                    with zipfile.ZipFile(nested_zip_path) as nzf:
+                        nzf.extractall(nested_extract)
+                        for nested_name in nzf.namelist():
+                            if nested_name.lower().endswith(".fit"):
+                                fit_path = nested_extract / nested_name
+                                task = process_activity_file.delay(str(fit_path), current_user.id, "fit")
+                                task_ids.append(task.id)
+                except zipfile.BadZipFile:
+                    pass
 
     # Queue health/wellness data extraction
     health_task = process_garmin_health_zip.delay(str(dest), current_user.id)