feat: Add currency management feature with CRUD operations

- Introduced a new template for currency overview and management (`currencies.html`).
- Updated footer to include attribution to AllYouCanGET.
- Added "Currencies" link to the main navigation header.
- Implemented end-to-end tests for currency creation, update, and activation toggling.
- Created unit tests for currency API endpoints, including creation, updating, and activation toggling.
- Added a fixture to seed default currencies for testing.
- Enhanced database setup tests to ensure proper seeding and migration handling.

tests/unit/test_currencies.py

@@ -0,0 +1,101 @@
+from typing import Dict
+
+import pytest
+
+from models.currency import Currency
+
+
+@pytest.fixture(autouse=True)
+def _cleanup_currencies(db_session):
+    db_session.query(Currency).delete()
+    db_session.commit()
+    yield
+    db_session.query(Currency).delete()
+    db_session.commit()
+
+
+def _assert_currency(payload: Dict[str, object], code: str, name: str, symbol: str | None, is_active: bool) -> None:
+    assert payload["code"] == code
+    assert payload["name"] == name
+    assert payload["is_active"] is is_active
+    if symbol is None:
+        assert payload["symbol"] is None
+    else:
+        assert payload["symbol"] == symbol
+
+
+def test_list_returns_default_currency(api_client, db_session):
+    response = api_client.get("/api/currencies/")
+    assert response.status_code == 200
+    data = response.json()
+    assert any(item["code"] == "USD" for item in data)
+
+
+def test_create_currency_success(api_client, db_session):
+    payload = {"code": "EUR", "name": "Euro", "symbol": "€", "is_active": True}
+    response = api_client.post("/api/currencies/", json=payload)
+    assert response.status_code == 201
+    data = response.json()
+    _assert_currency(data, "EUR", "Euro", "€", True)
+
+    stored = db_session.query(Currency).filter_by(code="EUR").one()
+    assert stored.name == "Euro"
+    assert stored.symbol == "€"
+    assert stored.is_active is True
+
+
+def test_create_currency_conflict(api_client, db_session):
+    api_client.post(
+        "/api/currencies/",
+        json={"code": "CAD", "name": "Canadian Dollar",
+              "symbol": "$", "is_active": True},
+    )
+    duplicate = api_client.post(
+        "/api/currencies/",
+        json={"code": "CAD", "name": "Canadian Dollar",
+              "symbol": "$", "is_active": True},
+    )
+    assert duplicate.status_code == 409
+
+
+def test_update_currency_fields(api_client, db_session):
+    api_client.post(
+        "/api/currencies/",
+        json={"code": "GBP", "name": "British Pound",
+              "symbol": "£", "is_active": True},
+    )
+
+    response = api_client.put(
+        "/api/currencies/GBP",
+        json={"name": "Pound Sterling", "symbol": "£", "is_active": False},
+    )
+    assert response.status_code == 200
+    data = response.json()
+    _assert_currency(data, "GBP", "Pound Sterling", "£", False)
+
+
+def test_toggle_currency_activation(api_client, db_session):
+    api_client.post(
+        "/api/currencies/",
+        json={"code": "AUD", "name": "Australian Dollar",
+              "symbol": "A$", "is_active": True},
+    )
+
+    response = api_client.patch(
+        "/api/currencies/AUD/activation",
+        json={"is_active": False},
+    )
+    assert response.status_code == 200
+    data = response.json()
+    _assert_currency(data, "AUD", "Australian Dollar", "A$", False)
+
+
+def test_default_currency_cannot_be_deactivated(api_client, db_session):
+    api_client.get("/api/currencies/")
+    response = api_client.patch(
+        "/api/currencies/USD/activation",
+        json={"is_active": False},
+    )
+    assert response.status_code == 400
+    assert response.json()[
+        "detail"] == "The default currency cannot be deactivated."