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/sdk.py

@@ -28,7 +28,12 @@ from app.identity.deps import current_user, require_csrf
 from app.indexer import blobstore
 from app.models import User
 from app.outbox import service as outbox_service
-from app.plugins.actions import PluginActions, PluginMessages, PluginThreads
+from app.plugins.actions import (
+    PluginAccounts,
+    PluginActions,
+    PluginMessages,
+    PluginThreads,
+)
 from app.plugins.db import plugin_session
 from app.plugins.http import PluginHttp
 from app.plugins.kv import PluginKvStore
@@ -153,6 +158,11 @@ class PluginSdk:
         ``move_to_special`` (one message into e.g. Junk)."""
         return PluginActions()
 
+    @property
+    def accounts(self) -> PluginAccounts:
+        """Cross-user account listing for scheduled jobs (`live()`)."""
+        return PluginAccounts()
+
     @property
     def threads(self) -> PluginThreads:
         """Read-only thread queries (e.g. pruning stale references)."""