fix: don't enforce write-time transfer rules when reading transactions (#7)

TransactionRead inherited TransactionBase's model_validator, so the rules a
new row must satisfy were also applied on the way out, to rows already in
the database. A row can stop satisfying them through no fault of its own:
transactions.transfer_account_id is ON DELETE SET NULL, so deleting an
account leaves every transfer that pointed at it without a destination.

The result was that deleting an account could 500 the entire transactions
list — permanently, and with the list gone there was no way in the UI to
find and delete the offending row.

The fields move to TransactionFields, which reading uses; TransactionBase
keeps the invariants for the write path. The frontend already tolerates a
missing destination (TransactionsTable renders the transfer suffix only when
transfer_account_id is set, and the edit modal makes you pick a destination
before saving), so such a row is now visible, editable and deletable
instead of fatal.

backend/tests/test_transactions.py

@@ -378,3 +378,26 @@ async def test_update_rejects_a_category_on_a_transfer(client: AsyncClient, acco
     )
 
     assert resp.status_code == 400
+
+
+async def test_a_transfer_whose_destination_account_was_deleted_can_still_be_listed(
+    client: AsyncClient, account_id
+):
+    """Deleting an account nulls transfer_account_id on the transfers that
+    pointed at it (the FK is ON DELETE SET NULL), leaving a row that create
+    would reject. Refusing to *serialize* such a row takes the whole
+    transactions list down with a 500 — and with the list gone there is no
+    way left in the UI to delete the row and recover. Reading has to stay
+    possible; only writing is guarded."""
+    other = (await client.post("/accounts", json={"name": "Savings", "type": "savings"})).json()
+    created = await client.post(
+        "/transactions", json=_txn(account_id, type="transfer", transfer_account_id=other["id"], category_id=None)
+    )
+    txn_id = created.json()["id"]
+    assert (await client.delete(f"/accounts/{other['id']}")).status_code == 204
+
+    listing = await client.get("/transactions")
+
+    assert listing.status_code == 200
+    assert txn_id in [item["id"] for item in listing.json()["items"]]
+    assert (await client.delete(f"/transactions/{txn_id}")).status_code == 204