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

requirements.txt

@@ -0,0 +1,8 @@
+caldav==1.3.9
+icalendar==5.0.11
+urllib3==2.1.0
+beautifulsoup4>=4.12
+
+# Testing
+pytest>=8.0
+pytest-mock>=3.12