fix: handle unique constraint violation in manual fetch endpoint

When a duplicate timestamp occurs (rare but possible), return the
existing record instead of failing with a 500 error. This matches
the worker's ON CONFLICT DO NOTHING behavior.

Added test for duplicate timestamp handling.

backend/tests/test_price_history.py

@@ -241,6 +241,39 @@ class TestManualFetch:
         assert len(data) == 1
         assert data[0]["price"] == 87654.32
 
+    @pytest.mark.asyncio
+    async def test_returns_existing_record_on_duplicate_timestamp(
+        self, client_factory, admin_user
+    ):
+        """Verify duplicate timestamp returns existing record instead of error."""
+        fixed_timestamp = datetime(2024, 1, 15, 12, 0, 0, tzinfo=UTC)
+
+        # Seed an existing record with the same timestamp we'll get from the mock
+        async with client_factory.get_db_session() as db:
+            existing = PriceHistory(
+                source=SOURCE_BITFINEX,
+                pair=PAIR_BTC_EUR,
+                price=90000.0,
+                timestamp=fixed_timestamp,
+            )
+            db.add(existing)
+            await db.commit()
+            await db.refresh(existing)
+            existing_id = existing.id
+
+        # Mock fetch_btc_eur_price to return the same timestamp
+        with patch("routes.audit.fetch_btc_eur_price") as mock_fetch:
+            mock_fetch.return_value = (95000.0, fixed_timestamp)
+
+            async with client_factory.create(cookies=admin_user["cookies"]) as authed:
+                response = await authed.post("/api/audit/price-history/fetch")
+
+        # Should succeed and return the existing record
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == existing_id
+        assert data["price"] == 90000.0  # Original price, not the new one
+
 
 def create_mock_pool(mock_conn: AsyncMock) -> MagicMock:
     """Create a mock asyncpg pool with proper async context manager behavior."""