Add recurring series to events

A Series checkbox under When opens a panel for the schedule:
weekly on chosen weekdays, monthly by date, or monthly by weekday
position, every N weeks or months, with meeting times and an
optional meeting count. starts_on anchors the schedule and ends_on
bounds it, so effective_status needs no change.

Occurrences are derived, not stored. src/lib/eventSeries.ts builds
the schedule label shown on cards and the event page, and the
upcoming dates listed on the event page.

Migration 016 adds the columns; the CRUD engine gains a time type.

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

src/lib/adminSchema.d.ts

@@ -27,7 +27,8 @@ export type FieldWidget =
   | "checkbox"
   | "color"
   | "number"
-  | "date";
+  | "date"
+  | "time";
 
 /** A bare value, or [value, label]. */
 export type SelectOption = string | readonly [string, string];