feat: Implement currency management with models, routes, and UI updates; add backfill script for existing records

models/currency.py

@@ -0,0 +1,21 @@
+from sqlalchemy import Column, Integer, String, Boolean
+from sqlalchemy.orm import relationship
+from config.database import Base
+
+
+class Currency(Base):
+    __tablename__ = "currency"
+
+    id = Column(Integer, primary_key=True, index=True)
+    code = Column(String(3), nullable=False, unique=True, index=True)
+    name = Column(String(128), nullable=False)
+    symbol = Column(String(8), nullable=True)
+    is_active = Column(Boolean, nullable=False, default=True)
+
+    # reverse relationships (optional)
+    capex_items = relationship(
+        "Capex", back_populates="currency", lazy="select")
+    opex_items = relationship("Opex", back_populates="currency", lazy="select")
+
+    def __repr__(self):
+        return f"<Currency code={self.code} name={self.name} symbol={self.symbol}>"