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/routers/settings_router.py

@@ -29,6 +29,7 @@ class SettingsUpdate(BaseModel):
     bg_color:   Optional[str] = None
     private_event_visibility: Optional[str] = None
     group_visible_calendar_id: Optional[int] = None
+    default_reminder_minutes: Optional[int] = None  # null = off
 
 
 def _settings_dict(s: models.UserSettings) -> dict:
@@ -50,6 +51,7 @@ def _settings_dict(s: models.UserSettings) -> dict:
         "bg_color":   s.bg_color,
         "private_event_visibility": s.private_event_visibility or "busy",
         "group_visible_calendar_id": s.group_visible_calendar_id,
+        "default_reminder_minutes": s.default_reminder_minutes,
     }
 
 
@@ -92,7 +94,7 @@ def update_settings(
     # For these three override colours, an explicit null is meaningful
     # ("reset to default") and must be persisted as NULL. All other fields
     # keep the previous behaviour where a null/missing value is ignored.
-    NULLABLE_OVERRIDES = {"text_color", "line_color", "bg_color", "group_visible_calendar_id"}
+    NULLABLE_OVERRIDES = {"text_color", "line_color", "bg_color", "group_visible_calendar_id", "default_reminder_minutes"}
     update_data = data.model_dump(exclude_unset=True)
     for field, value in update_data.items():
         if field in NULLABLE_OVERRIDES: