Check confirmation/existence via scripthash history, not verbose replies (B-41)

poll_once and reconcile.py's existence check both called
blockchain.transaction.get(txid, verbose=True). Several Electrum server
implementations and versions reject the verbose flag outright
("verbose transactions are currently unsupported"), which would have
meant no confirmations and no reconciliation ever running against such
a server, read as a plain transport error. reconcile.py additionally
decided whether to abandon a transaction - releasing its funds - by
substring-matching the error text ("missing", "not found", ...), which
only works against ElectrumX's specific wording.

Both now ask blockchain.scripthash.get_history for the address that
owns every input of the transaction (a user's own address for a
bet/withdrawal, the pool address for a payout) and look for the txid in
the result: present with height > 0 means confirmed, present with
height <= 0 means still in the mempool, absent means the server
doesn't know it. get_history is a plain, universally-supported Electrum
method, and "not in the list" replaces the old substring-matching
entirely - no more guessing at error wording to decide whether to
release funds. History is cached per scripthash within one pass, since
every "payout" row shares the same pool address.

New app/tx/pending_address.py factors out own_address_for (the
address derivation was previously duplicated informally inside
tx/broadcast.py's signing context) so confirmation.py and reconcile.py
share one definition instead of two that could compute different
addresses for the same row.

tests/unit/test_confirmation.py and test_reconcile.py needed real User
rows and a master-key bootstrap they didn't have before, since address
derivation is now exercised for real rather than assumed. Suite grows
from 217 to 222 tests. BUGS.md moves B-41 to Previously fixed - no
Medium-severity finding remains open.

app/electrum/client.py

@@ -169,6 +169,17 @@ class ElectrumClient:
     async def listunspent(self, scripthash: str) -> list[dict]:
         return await self.request("blockchain.scripthash.listunspent", [scripthash])
 
+    async def get_history(self, scripthash: str) -> list[dict]:
+        """Every transaction touching `scripthash`, each as {"tx_hash", "height"} —
+        height > 0 means confirmed at that height, height <= 0 means still in the
+        mempool. Used instead of blockchain.transaction.get's verbose=True mode
+        for confirmation/existence checks (B-41): several Electrum server
+        implementations and versions reject the verbose flag outright ("verbose
+        transactions are currently unsupported"), while get_history is a plain,
+        universally-supported method every server must implement.
+        """
+        return await self.request("blockchain.scripthash.get_history", [scripthash])
+
     async def broadcast(self, raw_tx_hex: str) -> str:
         return await self.request("blockchain.transaction.broadcast", [raw_tx_hex])