Let each person decide whether a shared calendar reminds them

reminders_enabled belongs to the calendar, so it belongs to its owner: whoever
shares a calendar decides whether it notifies everyone it reaches, and a
recipient's PUT is refused with 404 by the owner-only filter. For a group
calendar that is worse than it sounds — the creator owns it, so silencing it
for themselves would silence every member.

CalendarReminderPref follows CalendarColorPref exactly, which already solves
this for colours. Only local calendars are shareable (CalendarShare,
GroupCalendar and group_visible_calendar_id all point at local_calendars.id),
so no source discrimination is needed and the other four routers are untouched.
No migration: create_all makes new tables, which is why calendar_color_prefs
has no migration entry either.

A recipient without an override gets reminders on, rather than inheriting the
owner's choice. Muting is personal and should not spread. The new endpoint
decides owner-vs-override server-side, so the existing owner-only PUT stays the
guard it already is, and GET keeps returning reminders_enabled — now carrying
the effective value, so no client model has to change.

Birthday reminders were injected without consulting the flag at all, so a muted
birthday calendar still handed the schedulers something to fire.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/permissions.py

@@ -187,6 +187,17 @@ def readable_local_calendar_ids(db: Session, user: models.User) -> list[int]:
     return list(ids)
 
 
+def reminder_prefs_for(db: Session, user_id: int) -> dict[int, bool]:
+    """Map calendar_id -> the user's personal reminder flag for calendars they
+    don't own (any sharing path). Empty when the user set no overrides."""
+    return {
+        p.calendar_id: bool(p.enabled)
+        for p in db.query(models.CalendarReminderPref).filter(
+            models.CalendarReminderPref.user_id == user_id
+        )
+    }
+
+
 def color_prefs_for(db: Session, user_id: int) -> dict[int, str]:
     """Map calendar_id -> the user's personal colour for calendars they don't
     own (any sharing path). Empty when the user set no overrides."""