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/db/schemas.py

@@ -0,0 +1,58 @@
+"""Pydantic schemas for API and validation."""
+
+from typing import Literal
+
+from pydantic import BaseModel, ConfigDict
+
+
+class UserBase(BaseModel):
+    full_name: str
+    username: str | None = None
+    first_name: str | None = None
+    last_name: str | None = None
+
+
+class UserCreate(UserBase):
+    telegram_user_id: int
+
+
+class UserInDb(UserBase):
+    id: int
+    telegram_user_id: int
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class DutyBase(BaseModel):
+    user_id: int
+    start_at: str  # UTC, ISO 8601 with Z
+    end_at: str  # UTC, ISO 8601 with Z
+
+
+class DutyCreate(DutyBase):
+    pass
+
+
+class DutyInDb(DutyBase):
+    id: int
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class DutyWithUser(DutyInDb):
+    """Duty with full_name and event_type for calendar display.
+
+    event_type: only these values are returned; unknown DB values are mapped to "duty" in the API.
+    """
+
+    full_name: str
+    event_type: Literal["duty", "unavailable", "vacation"] = "duty"
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class CalendarEvent(BaseModel):
+    """External calendar event (e.g. holiday) for a single day."""
+
+    date: str  # YYYY-MM-DD
+    summary: str