Cap the number of inputs a transaction may spend (B-48)

select_utxos had no ceiling on input count, so an address fragmented into many
small deposits built an ever-larger transaction whose fee — deducted from the
amount being moved — eroded the bet's share of the pool or the withdrawn amount,
and past a few hundred inputs stopped being standard at all.

MAX_TX_INPUTS (50) now bounds the selection. Reaching the cap without covering
the target is reported as its own "too_many_inputs" code, distinct from having
no funds, with the cap carried in the error params for the 7 translations. The
payout path records the same distinction in its payout_failed audit reason.

tests/unit/test_bets.py

@@ -10,6 +10,7 @@ from app.db.base import Base
 from app.db.models import AuditLog, PendingTransaction, Round, RoundConfig, RoundParticipant, User, UtxoEvent
 from app.rounds.service import open_new_round_if_needed
 from app.wallet.hd import derive_user_address
+from app.wallet.psbt_builder import MAX_TX_INPUTS
 
 
 class FakeElectrumClient:
@@ -91,6 +92,35 @@ async def test_place_bet_rejects_insufficient_balance(session_factory):
             await place_bet(session, client, user)
 
 
+async def test_place_bet_reports_a_too_fragmented_balance_distinctly(session_factory):  # B-48
+    # 100 x 0.15 PLM = 15 PLM, plenty for a 10 PLM bet, but the 50 largest inputs
+    # only add up to 7.5 PLM — so the build must fail with its own code, not with
+    # the "you have no funds" one, and must carry the cap for the translation.
+    user_id = await _make_funded_user(session_factory, 20, 15_000_000)
+    async with session_factory() as session:
+        for i in range(99):
+            session.add(
+                UtxoEvent(
+                    user_id=user_id,
+                    txid=f"{i:064x}",
+                    vout=0,
+                    amount_sats=15_000_000,
+                    confirmed_height=100,
+                )
+            )
+        await session.commit()
+    client = FakeElectrumClient()
+
+    async with session_factory() as session:
+        user = await session.get(User, user_id)
+        with pytest.raises(BetError) as excinfo:
+            await place_bet(session, client, user)
+
+    assert excinfo.value.code == "too_many_inputs"
+    assert excinfo.value.params == {"max_inputs": MAX_TX_INPUTS}
+    assert not client.broadcasted
+
+
 async def test_place_bet_rejects_second_bet_same_round(session_factory):
     user_id = await _make_funded_user(session_factory, 2, 3_000_000_000)
     client = FakeElectrumClient()