Add IMAP to Nextcloud Calendar importer

- IMAP mailbox monitoring with SSL/TLS, STARTTLS and plain connection support
- ICS attachment extraction from emails and import to Nextcloud via CalDAV
- Create or update calendar events by UID to avoid duplicates
- Event filtering by summary (contains, regex or exact match, with inversion)
- Deduplication by regex pattern extracted from event summaries
- Move or mark processed emails as seen after import
- Continuous polling mode and one-time run mode
- Fully configurable via environment variables
- Docker and Docker Compose support
- Test suite with pytest

docker-compose.yml

@@ -0,0 +1,8 @@
+services:
+  imap-calendar-importer:
+    build: .
+    container_name: imap-calendar-importer
+    restart: unless-stopped
+    env_file: ./.env
+    volumes:
+      - ./config:/app/config