Refactor database initialization and remove Alembic migrations

- Removed legacy Alembic migration files and consolidated schema management into a new Pydantic-backed initializer (`scripts/init_db.py`).
- Updated `main.py` to ensure the new DB initializer runs on startup, maintaining idempotency.
- Adjusted session management in `config/database.py` to prevent DetachedInstanceError.
- Introduced new enums in `models/enums.py` for better organization and clarity.
- Refactored various models to utilize the new enums, improving code maintainability.
- Enhanced middleware to handle JSON validation more robustly, ensuring non-JSON requests do not trigger JSON errors.
- Added tests for middleware and enums to ensure expected behavior and consistency.
- Updated changelog to reflect significant changes and improvements.

models/simulation_parameter.py

@@ -1,9 +1,10 @@
 from __future__ import annotations
 
 from datetime import datetime
-from enum import Enum
 from typing import TYPE_CHECKING
 
+from .enums import DistributionType, ResourceType, StochasticVariable
+
 from sqlalchemy import (
     JSON,
     DateTime,
@@ -17,22 +18,11 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.sql import func
 
 from config.database import Base
-from .metadata import ResourceType, StochasticVariable
 
 if TYPE_CHECKING:  # pragma: no cover
     from .scenario import Scenario
 
 
-class DistributionType(str, Enum):
-    """Supported stochastic distribution families for simulations."""
-
-    NORMAL = "normal"
-    TRIANGULAR = "triangular"
-    UNIFORM = "uniform"
-    LOGNORMAL = "lognormal"
-    CUSTOM = "custom"
-
-
 class SimulationParameter(Base):
     """Probability distribution settings for scenario simulations."""
 
@@ -44,13 +34,13 @@ class SimulationParameter(Base):
     )
     name: Mapped[str] = mapped_column(String(255), nullable=False)
     distribution: Mapped[DistributionType] = mapped_column(
-        SQLEnum(DistributionType), nullable=False
+        SQLEnum(DistributionType, name="distributiontype", create_type=False), nullable=False
     )
     variable: Mapped[StochasticVariable | None] = mapped_column(
-        SQLEnum(StochasticVariable), nullable=True
+        SQLEnum(StochasticVariable, name="stochasticvariable", create_type=False), nullable=True
     )
     resource_type: Mapped[ResourceType | None] = mapped_column(
-        SQLEnum(ResourceType), nullable=True
+        SQLEnum(ResourceType, name="resourcetype", create_type=False), nullable=True
     )
     mean_value: Mapped[float | None] = mapped_column(
         Numeric(18, 4), nullable=True)