Phase 7 follow-up: 1-month body retention and actions_log cap (AUD-75)

Implements the AGENTS.md §7 data rule: keep message bodies for one month, and
size-cap the append-only action log.

- internal/store/retention.go: Store.Retain clears messages.body_text older
  than the window (metadata stays) and deletes actions_log rows beyond the
  newest N, in one transaction.
- internal/config: EMAIL_AGENT_RETENTION_DAYS (30) and
  EMAIL_AGENT_ACTIONS_LOG_MAX_ROWS (50000).
- cmd/email-agent: bounded -retain flag for the n8n schedule.
- Tests for body clearing, metadata retention, log cap, and disabled policy.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docker-compose.yml

@@ -62,6 +62,10 @@ services:
       APPRISE_URLS: ${APPRISE_URLS:-}
       APPRISE_TOKEN: ${APPRISE_TOKEN:-}
       EMAIL_AGENT_SUMMARY_TO: ${EMAIL_AGENT_SUMMARY_TO:-samuelrolfe@gmail.com}
+      # Retention (AGENTS.md §7): `-retain` clears message bodies older than
+      # this window and caps the append-only actions_log.
+      EMAIL_AGENT_RETENTION_DAYS: ${EMAIL_AGENT_RETENTION_DAYS:-30}
+      EMAIL_AGENT_ACTIONS_LOG_MAX_ROWS: ${EMAIL_AGENT_ACTIONS_LOG_MAX_ROWS:-50000}
 
     volumes:
       - email_agent_data:/data