sync-v2 step 2: extract AccountRunner

Move the per-account INBOX-IDLE + secondary-poll loops out of the supervisor
into a self-contained AccountRunner (start/stop/teardown). The startup supervisor
now launches one runner per live account. Pure refactor, no behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/sync/runner.py

@@ -0,0 +1,68 @@
+"""Per-account sync runner (refactor §4).
+
+An ``AccountRunner`` owns the long-lived sync loops for a single account and
+exposes a small ``start`` / ``stop`` lifecycle. This is a pure extraction of the
+loops that previously lived inline in the supervisor — behavior is unchanged
+(INBOX IDLE + secondary CONDSTORE poll, each self-healing with backoff).
+"""
+
+from __future__ import annotations
+
+import asyncio
+import contextlib
+import logging
+import uuid
+from collections.abc import Awaitable, Callable
+
+from app.sync.engine import live_loop, secondary_poll_loop
+
+logger = logging.getLogger(__name__)
+
+_RECONNECT_BACKOFF = 30.0
+
+
+class AccountRunner:
+    """Owns the sync loops for one account until stopped."""
+
+    def __init__(self, account_id: uuid.UUID) -> None:
+        self.account_id = account_id
+        self._stop = asyncio.Event()
+        self._tasks: list[asyncio.Task[None]] = []
+
+    def start(self) -> None:
+        """Launch the account's sync loops (idempotent-ish: call once)."""
+        self._stop.clear()
+        self._tasks = [
+            asyncio.create_task(self._supervise(self._run_live, "live")),
+            asyncio.create_task(self._supervise(self._run_secondary, "secondary")),
+        ]
+
+    def running(self) -> bool:
+        return any(not t.done() for t in self._tasks)
+
+    async def stop(self) -> None:
+        """Signal and cancel the loops, waiting for teardown."""
+        self._stop.set()
+        for task in self._tasks:
+            task.cancel()
+        if self._tasks:
+            await asyncio.gather(*self._tasks, return_exceptions=True)
+        self._tasks = []
+
+    async def _run_live(self) -> None:
+        await live_loop(self.account_id, "INBOX", self._stop)
+
+    async def _run_secondary(self) -> None:
+        await secondary_poll_loop(self.account_id, self._stop)
+
+    async def _supervise(self, run: Callable[[], Awaitable[None]], label: str) -> None:
+        """Run a loop, restarting it (with backoff) until stopped."""
+        while not self._stop.is_set():
+            try:
+                await run()
+            except asyncio.CancelledError:
+                raise
+            except Exception:
+                logger.exception("%s loop for account %s crashed; retrying", label, self.account_id)
+                with contextlib.suppress(TimeoutError):
+                    await asyncio.wait_for(self._stop.wait(), timeout=_RECONNECT_BACKOFF)