Calendar reminders as push notifications

A job every minute finds the event reminders that fell due — repeating events
and moved occurrences expanded as the calendar view does — and pushes each
once to every device with notifications on (sdk.push, new: plugins can notify
through the new-mail push delivery). A reminder missed while the worker was down
is still sent within 10 minutes; cancelled events and declined invitations stay
quiet. Title, then "In 15 minutes, 14:00 · Room 2" in the user's zone and clock
format; it stays until dismissed and shows even with the app open; clicking it
opens that day (/p/calendar?date=…, new deep link).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/push/service.py

@@ -200,6 +200,26 @@ async def notify(user_id: uuid.UUID, message_ids: list[uuid.UUID]) -> int:
     pushes were delivered to push services."""
     from app.db import session_scope
 
+    async with session_scope() as db:
+        if not await db.scalar(
+            select(PushSubscription.id).where(PushSubscription.user_id == user_id).limit(1)
+        ):
+            return 0
+        messages = await _news(db, user_id, message_ids)
+        if not messages:
+            return 0
+        payloads = _payloads(messages, await _unread(db, user_id))
+    return await deliver(user_id, payloads)
+
+
+async def deliver(user_id: uuid.UUID, payloads: list[dict[str, object]]) -> int:
+    """Push ``payloads`` to every device of the user that is still signed in
+    (for new mail, and for plugins: calendar reminders); returns how many were
+    handed to push services. Devices that are gone are forgotten."""
+    from app.db import session_scope
+
+    if not payloads:
+        return 0
     async with session_scope() as db:
         # Plain values: the rows expire when the key pair is first saved below.
         subs = [
@@ -210,10 +230,6 @@ async def notify(user_id: uuid.UUID, message_ids: list[uuid.UUID]) -> int:
         ]
         if not subs:
             return 0
-        messages = await _news(db, user_id, message_ids)
-        if not messages:
-            return 0
-        payloads = _payloads(messages, await _unread(db, user_id))
         key = await vapid(db)
         subject = _subject(await db.get(AppSettings, 1))