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/main.py

@@ -102,6 +102,20 @@ def _migrate():
         except Exception:
             pass
 
+        try:
+            conn.execute(text("ALTER TABLE local_events ADD COLUMN reminders TEXT"))
+            conn.commit()
+            logging.info("Migration: added reminders to local_events")
+        except Exception:
+            pass
+
+        try:
+            conn.execute(text("ALTER TABLE user_settings ADD COLUMN default_reminder_minutes INTEGER"))
+            conn.commit()
+            logging.info("Migration: added default_reminder_minutes to user_settings")
+        except Exception:
+            pass
+
         try:
             conn.execute(text("ALTER TABLE user_settings ADD COLUMN month_divider_color VARCHAR(7) DEFAULT '#7090c0'"))
             conn.commit()