feat(outbox): implement concurrent claim mechanism with UPDATE RETURNING + SKIP LOCKED

- Add migration 0004 to introduce 'claiming' status and timeout index
- Add StatusClaiming to platformevent domain and allow it in Validate()
- Rewrite ListDue as transactional UPDATE ... RETURNING with FOR UPDATE SKIP LOCKED
- Add ReleaseStaleClaims to reset expired claiming events back to retrying
- Worker Start() now runs a 30s ticker for stale claim recovery (5m timeout)
- Update stubEventStore in tests to satisfy new EventStore interface

Refs: D-02

db/migration/0004_outbox_claiming_status.up.sql

@@ -0,0 +1,11 @@
+-- Add 'claiming' status to outbox CHECK constraint and add claim timeout index
+
+ALTER TABLE cs_platform_event_outbox
+DROP CONSTRAINT IF EXISTS chk_cs_platform_event_outbox_status;
+
+ALTER TABLE cs_platform_event_outbox
+ADD CONSTRAINT chk_cs_platform_event_outbox_status
+CHECK (status IN ('pending','retrying','delivered','dead_letter','claiming'));
+
+CREATE INDEX IF NOT EXISTS idx_cs_platform_event_outbox_claiming_timeout
+ON cs_platform_event_outbox(status, updated_at);