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

Dockerfile

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+FROM python:3.11-slim
+
+# Set working directory
+WORKDIR /app
+
+# Install dependencies
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application
+COPY imap_calendar_importer.py .
+
+# Run as non-root user
+RUN useradd -m -u 1000 appuser && chown -R appuser:appuser /app
+USER appuser
+
+# Default command
+CMD ["python", "-u", "imap_calendar_importer.py"]