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/tx/reconcile.py

@@ -36,7 +36,9 @@ from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 from app.audit.log import write_audit_log
 from app.db.models import PendingTransaction, Round, RoundParticipant, UtxoEvent, Withdrawal
 from app.electrum.client import ElectrumClient
+from app.electrum.scripthash import address_to_scripthash
 from app.rounds.events import broadcaster
+from app.tx.pending_address import own_address_for
 from app.wallet.balance import recompute_balance
 
 logger = logging.getLogger(__name__)
@@ -72,22 +74,21 @@ class PendingTransactionReconciler:
             await asyncio.sleep(_POLL_INTERVAL_SECONDS)
 
 
-async def _tx_exists_on_chain(client: ElectrumClient, txid: str) -> bool:
-    """True if the server knows this txid at all (mempool or mined). An error reply
-    means "unknown", which is the answer we're looking for; a transport failure is
-    *not* — that raises, and the caller leaves the row alone until next time."""
-    try:
-        tx = await client.get_transaction(txid, verbose=True)
-    except Exception as exc:
-        message = str(exc).lower()
-        if "missing" in message or "not found" in message or "no such" in message or "unknown" in message:
-            return False
-        raise
-    return bool(tx)
-
-
 async def reconcile_once(session_factory: async_sessionmaker, client: ElectrumClient) -> int:
-    """Returns how many rows were resolved (promoted or abandoned)."""
+    """Returns how many rows were resolved (promoted or abandoned).
+
+    Existence is decided by checking whether a row's own address's history
+    (blockchain.scripthash.get_history) includes its txid at all — mempool or
+    mined — rather than asking blockchain.transaction.get for a verbose reply
+    (B-41): several Electrum server implementations and versions reject the
+    verbose flag outright, and the previous substring-matching on the error
+    text (looking for "missing", "not found", ...) was fragile as the basis for
+    a decision that releases funds. A transport failure fetching history still
+    raises and leaves the row alone until next time — get_history not
+    returning our txid is the only thing that means "gone". History is cached
+    per scripthash within one pass, since every "payout" row shares the same
+    pool address.
+    """
     now = datetime.now(timezone.utc)
     async with session_factory() as session:
         rows = (
@@ -95,16 +96,25 @@ async def reconcile_once(session_factory: async_sessionmaker, client: ElectrumCl
                 select(PendingTransaction).where(PendingTransaction.status.in_(("building", "pending")))
             )
         ).all()
-        candidates = [
-            (row.id, row.status, row.current_txid)
-            for row in rows
-            if _is_due(row, now)
-        ]
+        candidates = []
+        for row in rows:
+            if not _is_due(row, now):
+                continue
+            try:
+                address = await own_address_for(session, row.kind, row.user_id)
+                scripthash = address_to_scripthash(address)
+            except Exception:
+                logger.exception("could not derive the address for pending_transaction %s", row.id)
+                continue
+            candidates.append((row.id, row.status, row.current_txid, scripthash))
 
     resolved = 0
-    for row_id, status, txid in candidates:
+    history_cache: dict[str, list[dict]] = {}
+    for row_id, status, txid, scripthash in candidates:
         try:
-            exists = await _tx_exists_on_chain(client, txid)
+            if scripthash not in history_cache:
+                history_cache[scripthash] = await client.get_history(scripthash)
+            exists = any(entry.get("tx_hash") == txid for entry in history_cache[scripthash])
         except Exception:
             # Transport/server problem — say nothing about this tx and try again on
             # the next pass rather than abandoning a tx that may be perfectly alive.