Never roll back an ambiguous broadcast as if it had been refused (B-76, TX-04, TX-05, TX-20, TX-21, BET-16, BET-17, BET-23, WDR-11, WDR-12, WDR-16, API-13)

place_bet and request_withdrawal released everything on any exception from the
broadcast, including the 15s timeout and a connection dropped after the write. The
tx may have propagated: a bet's sats landed in the pool with no participant, a
withdrawal left while its row said failed and its coins were spendable again.

- client: ElectrumRpcError (subclass of ElectrumError) is raised only for a reply
  carrying a JSON-RPC `error`; timeouts, drops and write errors stay plain
  ElectrumError.
- tx/outcome.py:broadcast_with_verdict sorts a broadcast into accepted / refused /
  uncertain. A refusal is verified first: ElectrumListener.transaction_known asks
  blockchain.transaction.get of the active server and every other one (10s each) —
  any server returning the tx means accepted; the active server plus
  MIN_QUORUM_RESPONSES others all denying it means refused (ALLOW_NO_QUORUM keeps
  the reduced rule); too few answers is uncertain. "already known" wording is only a
  shortcut that skips the check. With no network to ask, a refusal is uncertain.
- place_bet / request_withdrawal / _trigger_payout use it: only a verified refusal
  releases; uncertain leaves the rows building for the reconciler, publishes SSE and
  POST /bets and POST /withdrawals answer 202 with status building; "already known"
  promotes a payout (payout_sent) like any accepted broadcast.
- An accepted transaction is fanned out to every other server
  (ElectrumListener.fan_out_transaction, best effort, off the caller's path). The RBF
  replacement in bump_fee is left for B-77, which rewrites that function.

Tests changed because the spec changed: the existing "refused broadcast" tests raised
a bare RuntimeError (now an uncertain outcome) and are made explicit refusals
(rpc_error) with a network that denies the tx; direct route calls pass a Response;
FakePayoutListener gains the network surface.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/unit/fake_network.py

@@ -0,0 +1,26 @@
+"""Test doubles for what the services ask of the network (B-76 and after)."""
+
+from app.electrum.client import ElectrumRpcError
+
+
+def rpc_error(message: str, code: int = -26) -> ElectrumRpcError:
+    """A server that answered with a JSON-RPC `error` object — an explicit refusal."""
+    return ElectrumRpcError({"code": code, "message": message})
+
+
+class FakeNetwork:
+    """The ElectrumListener surface a broadcast needs: `known` is what
+    `transaction_known` answers (True: some server has it, False: every server denies
+    it, None: too few answers to tell); fanned-out transactions are recorded."""
+
+    def __init__(self, known: bool | None = False):
+        self.known = known
+        self.asked: list[str] = []
+        self.fanned_out: list[str] = []
+
+    async def transaction_known(self, txid: str) -> bool | None:
+        self.asked.append(txid)
+        return self.known
+
+    def fan_out_transaction(self, raw_tx_hex: str) -> None:
+        self.fanned_out.append(raw_tx_hex)