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/session.py

@@ -0,0 +1,57 @@
+"""SQLAlchemy engine and session factory.
+
+Engine and session factory are cached globally per process. Only one DATABASE_URL
+is effectively used for the process lifetime. Using a different URL later (e.g. in
+tests with in-memory SQLite) would still use the first engine. To use a different
+URL in tests, set env (e.g. DATABASE_URL) before the first import of this module, or
+clear _engine and _SessionLocal in test fixtures. Prefer session_scope() for all
+callers so sessions are always closed and rolled back on error.
+"""
+
+from contextlib import contextmanager
+from typing import Generator
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session, sessionmaker
+
+
+_engine = None
+_SessionLocal = None
+
+
+@contextmanager
+def session_scope(database_url: str) -> Generator[Session, None, None]:
+    """Context manager: yields a session, rolls back on exception, closes on exit."""
+    session = get_session(database_url)
+    try:
+        yield session
+    except Exception:
+        session.rollback()
+        raise
+    finally:
+        session.close()
+
+
+def get_engine(database_url: str):
+    global _engine
+    if _engine is None:
+        _engine = create_engine(
+            database_url,
+            connect_args={"check_same_thread": False}
+            if "sqlite" in database_url
+            else {},
+            echo=False,
+        )
+    return _engine
+
+
+def get_session_factory(database_url: str) -> sessionmaker[Session]:
+    global _SessionLocal
+    if _SessionLocal is None:
+        engine = get_engine(database_url)
+        _SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+    return _SessionLocal
+
+
+def get_session(database_url: str) -> Session:
+    return get_session_factory(database_url)()