feat: add resumable per-user sync pipeline

app/sync/states.py

@@ -0,0 +1,12 @@
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class SyncOutcome:
+    user_id: int
+    status: str
+    discovered: int
+    imported: int
+    skipped: int
+    failed: int
+    message: str | None = None