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/research.md

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+# Research: Auto-Delete Expired Events
+
+**Feature**: 013-auto-delete-expired | **Date**: 2026-03-09
+
+## Deletion Strategy
+
+- **Decision**: Direct native SQL DELETE query via Spring Data JPA `@Query`.
+- **Rationale**: Simplest approach. No entity loading overhead. The existing `ON DELETE CASCADE` FK constraint on `fk_rsvps_event_id` (migration `003-create-rsvps-table.xml`) handles cascading deletion of RSVPs automatically. The existing `idx_events_expiry_date` index ensures the WHERE clause is efficient.
+- **Alternatives considered**:
+  - JPA repository `deleteAll(findExpired())`: Loads entities into memory first, unnecessary overhead.
+  - Database-level cron (`pg_cron`): Less portable, adds external dependency.
+  - Soft delete with lazy cleanup: Over-engineered for fete's scale and privacy goals.
+  - No deletion (filter only): Contradicts privacy-by-design principle.
+
+## Scheduling Mechanism
+
+- **Decision**: Spring `@Scheduled(cron = ...)` annotation.
+- **Rationale**: Already available in Spring Boot, no additional dependencies. Simple, declarative, well-tested.
+- **Alternatives considered**:
+  - Quartz Scheduler: Too heavy for a single daily job.
+  - External cron (OS-level): Requires separate process management, harder to test.
+
+## Transaction Behavior
+
+- **Decision**: Single transaction for the DELETE statement.
+- **Rationale**: A single `DELETE FROM events WHERE expiry_date < CURRENT_DATE` is atomic. If the DB connection drops mid-execution, the transaction rolls back and no events are partially deleted. The next run picks up all expired events.
+
+## Enabling @Scheduled
+
+- **Decision**: Add `@EnableScheduling` to `FeteApplication.java`.
+- **Rationale**: Simplest approach. Only one scheduled job exists, no need for a separate config class.