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

.env.sample

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+# IMAP Configuration
+IMAP_SERVER=imap.example.com
+IMAP_PORT=993
+IMAP_EMAIL=your-email@example.com
+IMAP_PASSWORD=your-password
+IMAP_MAILBOX=INBOX
+IMAP_PROCESSED_FOLDER=Processed
+
+# IMAP Security Options
+# For SSL/TLS (port 993): IMAP_USE_SSL=true, IMAP_USE_STARTTLS=false
+# For STARTTLS (port 143): IMAP_USE_SSL=false, IMAP_USE_STARTTLS=true
+# For plain (NOT RECOMMENDED): IMAP_USE_SSL=false, IMAP_USE_STARTTLS=false
+IMAP_USE_SSL=true
+IMAP_USE_STARTTLS=false
+# Set to false to accept self-signed certificates
+IMAP_VERIFY_SSL=true
+
+# Nextcloud CalDAV Configuration
+CALDAV_URL=https://your-nextcloud.com/remote.php/dav
+CALDAV_USERNAME=your-nextcloud-username
+CALDAV_PASSWORD=your-nextcloud-password
+CALENDAR_NAME=personal
+# Set to false to accept self-signed certificates
+CALDAV_VERIFY_SSL=true
+
+# Processing Options
+MARK_AS_SEEN=true
+MOVE_PROCESSED=false
+CHECK_INTERVAL=300
+RUN_ONCE=false
+
+# Summary Filter Options
+# Leave SUMMARY_FILTER empty to import all events
+# Examples:
+#   SUMMARY_FILTER=Meeting          (imports only events with "Meeting" in title)
+#   SUMMARY_FILTER=^Team.*          (regex: imports events starting with "Team")
+#   SUMMARY_FILTER=Dentist          (exact match with SUMMARY_FILTER_MODE=exact)
+SUMMARY_FILTER=
+SUMMARY_FILTER_MODE=contains
+INVERT_FILTER=false
+
+# Deduplication Options
+# Use these to replace existing events based on a pattern in the SUMMARY field
+# This is useful when the same event (e.g., delivery updates) gets different UIDs
+# Examples:
+#   DEDUPE_BY_SUMMARY_PATTERN=Order (\d+)     (extracts order number from "Ocado Delivery (Order 123456)")
+#   DEDUPE_BY_SUMMARY_PATTERN=Booking #(\w+)  (extracts booking reference)
+DEDUPE_ENABLED=false
+DEDUPE_BY_SUMMARY_PATTERN=
+
+# Logging
+LOG_LEVEL=INFO