feat: per-event reminders + default reminder setting (server)

local_events gains a `reminders` TEXT column (comma-separated minutes-before-
start, like exdate); EventCreate/EventUpdate accept a `reminders: [int]` list
and build_local_event_dict emits it back as a list. user_settings gains
`default_reminder_minutes` (nullable int, null = off), exposed/updatable via
/api/settings (explicit null persists as off). Migrations added in _migrate().
Clients (iOS/Android) schedule the OS notifications locally from these.

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

backend/local_events_util.py

@@ -128,6 +128,7 @@ def build_local_event_dict(
         "type": "local",
         "creator": creator,
         "private": bool(ev.is_private),
+        "reminders": [int(x) for x in (ev.reminders or "").split(",") if x.strip().lstrip("-").isdigit()],
     }
     if owner is not None:
         d["owner"] = owner