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/handlers/__init__.py

@@ -0,0 +1,17 @@
+"""Expose a single register_handlers(app) that registers all handlers."""
+
+from telegram.ext import Application
+
+from . import commands, errors, group_duty_pin, import_duty_schedule
+
+
+def register_handlers(app: Application) -> None:
+    app.add_handler(commands.start_handler)
+    app.add_handler(commands.help_handler)
+    app.add_handler(commands.set_phone_handler)
+    app.add_handler(import_duty_schedule.import_duty_schedule_handler)
+    app.add_handler(import_duty_schedule.handover_time_handler)
+    app.add_handler(import_duty_schedule.duty_schedule_document_handler)
+    app.add_handler(group_duty_pin.group_duty_pin_handler)
+    app.add_handler(group_duty_pin.pin_duty_handler)
+    app.add_error_handler(errors.error_handler)