Antispam: never classify during backfill

message.new fires for imported history as well as new arrivals, and EventBus
awaits hooks inline. As written, adding an account would have scored every
message in its backfill — filing years-old inbox mail into Junk on the server,
and adding ~70 ms of serialized HTTP per message to the backfill loop (twelve
minutes on a 10k-message account). It is also when the filter is least able to
judge, because Bayes has learned nothing yet.

The envelope now carries account_state, and the hook returns unless it is
"live". The backfill window belongs to the C2 learning bootstrap (§C.5), which
reads that history as training data instead of acting on it.

Tests pin the gating: backfilling accounts are not even read from the blob
store, non-inbox folders are ignored, a live inbox arrival still classifies, and
an unreachable engine still delivers.

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

backend/app/plugins/actions.py

@@ -23,7 +23,7 @@ from sqlalchemy import select
 from app.db import session_scope
 from app.events import get_event_bus
 from app.indexer import blobstore
-from app.models import Mailbox, Message, Thread
+from app.models import Account, Mailbox, Message, Thread
 
 
 class PluginMessages:
@@ -92,14 +92,15 @@ class PluginMessages:
         async with session_scope() as session:
             row = (
                 await session.execute(
-                    select(Message, Mailbox.special_use)
+                    select(Message, Mailbox.special_use, Account.state)
                     .join(Mailbox, Mailbox.id == Message.mailbox_id, isouter=True)
+                    .join(Account, Account.id == Message.account_id, isouter=True)
                     .where(Message.id == message_id, Message.user_id == user_id)
                 )
             ).first()
             if row is None:
                 return None
-            msg, special_use = row
+            msg, special_use, account_state = row
             sender = msg.from_addr[0] if msg.from_addr else {}
             return {
                 "message_id": str(msg.id),
@@ -108,6 +109,10 @@ class PluginMessages:
                 # with no SPECIAL-USE. A plugin that should only act on newly
                 # arrived mail checks this — `message.new` itself does not say.
                 "mailbox_special_use": special_use,
+                # "backfilling" | "live" | … — `message.new` fires for imported
+                # history too, so a plugin that should only react to genuinely
+                # new arrivals has to be able to tell the difference.
+                "account_state": account_state,
                 "subject": msg.subject or "",
                 "from_email": (sender.get("email") or "").lower(),
                 "from_name": sender.get("name") or "",