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/enums.py

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+from __future__ import annotations
+
+from enum import Enum
+
+
+class MiningOperationType(str, Enum):
+    """Supported mining operation categories."""
+
+    OPEN_PIT = "open_pit"
+    UNDERGROUND = "underground"
+    IN_SITU_LEACH = "in_situ_leach"
+    PLACER = "placer"
+    QUARRY = "quarry"
+    MOUNTAINTOP_REMOVAL = "mountaintop_removal"
+    OTHER = "other"
+
+
+class ScenarioStatus(str, Enum):
+    """Lifecycle states for project scenarios."""
+
+    DRAFT = "draft"
+    ACTIVE = "active"
+    ARCHIVED = "archived"
+
+
+class FinancialCategory(str, Enum):
+    """Enumeration of cost and revenue classifications."""
+
+    CAPITAL_EXPENDITURE = "capex"
+    OPERATING_EXPENDITURE = "opex"
+    REVENUE = "revenue"
+    CONTINGENCY = "contingency"
+    OTHER = "other"
+
+
+class DistributionType(str, Enum):
+    """Supported stochastic distribution families for simulations."""
+
+    NORMAL = "normal"
+    TRIANGULAR = "triangular"
+    UNIFORM = "uniform"
+    LOGNORMAL = "lognormal"
+    CUSTOM = "custom"
+
+
+class ResourceType(str, Enum):
+    """Primary consumables and resources used in mining operations."""
+
+    DIESEL = "diesel"
+    ELECTRICITY = "electricity"
+    WATER = "water"
+    EXPLOSIVES = "explosives"
+    REAGENTS = "reagents"
+    LABOR = "labor"
+    EQUIPMENT_HOURS = "equipment_hours"
+    TAILINGS_CAPACITY = "tailings_capacity"
+
+
+class CostBucket(str, Enum):
+    """Granular cost buckets aligned with project accounting."""
+
+    CAPITAL_INITIAL = "capital_initial"
+    CAPITAL_SUSTAINING = "capital_sustaining"
+    OPERATING_FIXED = "operating_fixed"
+    OPERATING_VARIABLE = "operating_variable"
+    MAINTENANCE = "maintenance"
+    RECLAMATION = "reclamation"
+    ROYALTIES = "royalties"
+    GENERAL_ADMIN = "general_admin"
+
+
+class StochasticVariable(str, Enum):
+    """Domain variables that typically require probabilistic modelling."""
+
+    ORE_GRADE = "ore_grade"
+    RECOVERY_RATE = "recovery_rate"
+    METAL_PRICE = "metal_price"
+    OPERATING_COST = "operating_cost"
+    CAPITAL_COST = "capital_cost"
+    DISCOUNT_RATE = "discount_rate"
+    THROUGHPUT = "throughput"