Keep the fee-rate ceiling below the node's maxfeerate (B-100)

MAX_FEE_RATE_SAT_VB was exactly sendrawtransaction's default maxfeerate
(10 000 sat/vB); with vsize estimated upwards a rate at the ceiling would be
refused. Now 5 000.

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

app/wallet/psbt_builder.py

@@ -30,8 +30,10 @@ DUST_LIMIT_SATS = 294
 # reason MIN_PASSWORD_LENGTH is shared in auth/security.py) and tx/broadcast.py's RBF
 # bump escalation, which refuses to bump a pending_transaction past this rate (B-32) —
 # without a ceiling, a stuck transaction's fee climbed by 1 sat/vB every bump forever,
-# eating further and further into the sender's change with no limit.
-MAX_FEE_RATE_SAT_VB = 10_000
+# eating further and further into the sender's change with no limit. B-100: half of the node's own
+# `sendrawtransaction` maxfeerate (0.1 PLM/kvB = 10 000 sat/vB, palladiumcore rpc/rawtransaction.cpp):
+# vsize is estimated upwards, so a rate sitting exactly on the node's limit would be refused.
+MAX_FEE_RATE_SAT_VB = 5_000
 
 # Ceiling on how many UTXOs one *user* transaction (bet, withdrawal) may spend (B-48).
 # Every extra input costs ~68 vbytes of fee, and that fee comes out of the amount being