fix: restore manual confirm_duplicate compatibility and precision safeguards

Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

tests/test_v1_portfolios_manual_ledger.py

@@ -558,6 +558,37 @@ def test_dividend_duplicate_requires_confirmation() -> None:
     assert confirmed.status_code == 201
 
 
+def test_confirm_duplicate_false_is_accepted_for_non_dividend_actions() -> None:
+    repo = _FakeRepo(_portfolio())
+    client = _make_client(repo)
+
+    invest = client.post(
+        f"/v1/portfolios/{_PORTFOLIO_ID}/ledger/manual",
+        json={"action": "INVEST", "amount": 100, "confirm_duplicate": False},
+        headers=_auth(),
+    )
+    assert invest.status_code == 201
+
+    buy = client.post(
+        f"/v1/portfolios/{_PORTFOLIO_ID}/ledger/manual",
+        json={"action": "BUY", "quantity": 1, "price": 10, "confirm_duplicate": False},
+        headers=_auth(),
+    )
+    assert buy.status_code == 201
+
+
+def test_confirm_duplicate_true_is_rejected_for_non_dividend_actions() -> None:
+    repo = _FakeRepo(_portfolio())
+    client = _make_client(repo)
+
+    invest = client.post(
+        f"/v1/portfolios/{_PORTFOLIO_ID}/ledger/manual",
+        json={"action": "INVEST", "amount": 100, "confirm_duplicate": True},
+        headers=_auth(),
+    )
+    assert invest.status_code == 422
+
+
 def test_dividend_reversal_decreases_profit_buffer() -> None:
     repo = _FakeRepo(_portfolio(buffer=200))
     client = _make_client(repo)