Refactor project structure and enhance Docker configuration

- Updated `.dockerignore` to exclude test and development artifacts, optimizing the Docker image size.
- Refactored `main.py` to delegate execution to `duty_teller.run.main()`, simplifying the entry point.
- Introduced a new `duty_teller` package to encapsulate core functionality, improving modularity and organization.
- Enhanced `pyproject.toml` to define a script for running the application, streamlining the execution process.
- Updated README documentation to reflect changes in project structure and usage instructions.
- Improved Alembic environment configuration to utilize the new package structure for database migrations.

duty_teller/utils/__init__.py

@@ -0,0 +1,21 @@
+"""Shared utilities: date/ISO helpers, user display names, handover time parsing."""
+
+from duty_teller.utils.dates import (
+    day_end_iso,
+    day_start_iso,
+    duty_to_iso,
+    parse_iso_date,
+    validate_date_range,
+)
+from duty_teller.utils.user import build_full_name
+from duty_teller.utils.handover import parse_handover_time
+
+__all__ = [
+    "day_start_iso",
+    "day_end_iso",
+    "duty_to_iso",
+    "parse_iso_date",
+    "validate_date_range",
+    "build_full_name",
+    "parse_handover_time",
+]