feat: add resumable per-user sync pipeline

tests/sync/fakes.py

@@ -0,0 +1,30 @@
+from app.garmin.uploader import UploadResult
+
+
+class FakeMyWhooshClient:
+    def __init__(self, activities, fit_bytes: bytes) -> None:
+        self.activities = activities
+        self.fit_bytes = fit_bytes
+        self.list_calls = 0
+        self.download_calls = 0
+
+    async def list_activities(self, email: str, password: str):
+        self.list_calls += 1
+        return list(self.activities)
+
+    async def download_fit(self, activity_file_id: str, email: str, password: str) -> bytes:
+        self.download_calls += 1
+        return self.fit_bytes
+
+
+class FakeGarminUploader:
+    def __init__(self, result: UploadResult | None = None, error: Exception | None = None) -> None:
+        self.result = result or UploadResult("imported", False, "g-1", {"activityId": "g-1"})
+        self.error = error
+        self.calls = 0
+
+    def import_fit(self, fit_path, mfa_code=None):
+        self.calls += 1
+        if self.error is not None:
+            raise self.error
+        return self.result