Auto-delete expired events via daily scheduled cleanup job

Adds a Spring @Scheduled job (daily at 03:00) that deletes all events
whose expiry_date is before CURRENT_DATE using a native SQL DELETE.
RSVPs are cascade-deleted via the existing FK constraint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/013-auto-delete-expired/data-model.md

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+# Data Model: Auto-Delete Expired Events
+
+**Feature**: 013-auto-delete-expired | **Date**: 2026-03-09
+
+## Existing Entities (no changes)
+
+### Event
+
+| Field | Type | Notes |
+|-------|------|-------|
+| id | BIGSERIAL | PK, internal |
+| event_token | UUID | Public identifier |
+| organizer_token | UUID | Organizer access |
+| title | VARCHAR(200) | Required |
+| description | VARCHAR(2000) | Optional |
+| date_time | TIMESTAMPTZ | Event date/time |
+| location | VARCHAR(500) | Optional |
+| expiry_date | DATE | **Deletion trigger** — indexed (`idx_events_expiry_date`) |
+| created_at | TIMESTAMPTZ | Auto-set |
+
+### RSVP
+
+| Field | Type | Notes |
+|-------|------|-------|
+| id | BIGSERIAL | PK, internal |
+| rsvp_token | UUID | Public identifier |
+| event_id | BIGINT | FK → events(id), **ON DELETE CASCADE** |
+| name | VARCHAR(100) | Guest name |
+
+## Deletion Behavior
+
+- `DELETE FROM events WHERE expiry_date < CURRENT_DATE` removes expired events.
+- RSVPs are automatically cascade-deleted by the FK constraint `fk_rsvps_event_id` with `ON DELETE CASCADE`.
+- No new tables, columns, or migrations required.
+
+## Indexes Used
+
+- `idx_events_expiry_date` on `events(expiry_date)` — ensures the cleanup query is efficient.