Antispam: bootstrap every account, not just the ones we happen to see go live

Warming Bayes hung on a single event — `account.state_changed` reaching "live".
That covers an account added from now on and nothing else. An account that went
live before this plugin existed never gets one; neither does one whose Junk
folder was empty at the time and filled up later, or one whose owner enabled
spam-folder sync afterwards. A worker restarting at the wrong moment loses it
too. The result was a feature that quietly required someone to run it by hand
per account, which is not a feature.

An hourly sweep now walks every live account and offers each a bootstrap. That
works because the per-account job was already cheap to repeat: it returns at once
for an account already done, and declines — without marking it done — when the
corpus is still too small to learn from. So the sweep simply keeps asking until
there is something worth learning, then does it once.

`s.accounts.live()` is the new SDK surface it needs: cross-user by design,
because a sweep runs for the instance rather than a request. Ids and state only,
so reading anything still goes through the user-scoped accessors.

One failing account cannot stop the sweep — the mailbox that raises is logged and
the rest still get their turn.

docs/plugins.md now says this in general terms next to `JobSpec`: a state-change
hook is one-shot, so pair it with a sweep when the work must eventually happen
for everything.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SqJYt7jD7oUEkoY68sBZo

backend/app/plugins/actions.py

@@ -162,6 +162,26 @@ class PluginMessages:
             }
 
 
+class PluginAccounts:
+    """Cross-user account queries, for scheduled jobs.
+
+    Deliberately not user-scoped: a sweep runs on behalf of the instance, not a
+    request, and needs to find work across every user. Nothing here returns
+    content — only ids and state — so a plugin still has to go through the
+    user-scoped accessors to read anything.
+    """
+
+    async def live(self) -> list[tuple[uuid.UUID, uuid.UUID]]:
+        """``(user_id, account_id)`` for every account finished with its initial
+        import. Accounts still backfilling are excluded: acting on one means
+        acting on a partial mailbox."""
+        async with session_scope() as session:
+            rows = await session.execute(
+                select(Account.user_id, Account.id).where(Account.state == "live")
+            )
+            return [(u, a) for u, a in rows]
+
+
 class PluginThreads:
     """Read-only, user-scoped thread queries (e.g. self-healing stale references)."""