rpcserver: wire ForwardingLog into router backend

diff --git a/config.go b/config.go
index d5f4a90..aaf748b 100644
--- a/config.go
+++ b/config.go
@@ -212,6 +212,10 @@ const (
 	// commitment.
 	defaultChannelCommitBatchSize = 10
 
+	// defaultFwdHistoryDeleteBatchSize is the default number of forwarding
+	// events deleted per database transaction when purging history.
+	defaultFwdHistoryDeleteBatchSize = 10_000
+
 	// defaultCoinSelectionStrategy is the coin selection strategy that is
 	// used by default to fund transactions.
 	defaultCoinSelectionStrategy = "largest"
@@ -418,6 +422,8 @@ type Config struct {
 
 	ChannelCommitBatchSize uint32 `long:"channel-commit-batch-size" description:"The maximum number of channel state updates that is accumulated before signing a new commitment."`
 
+	FwdHistoryDeleteBatchSize int `long:"fwd-history-delete-batch-size" description:"The number of forwarding events deleted per database transaction when running deletefwdhistory. Lower this on resource-constrained nodes to reduce lock contention (max: 50000)."`
+
 	KeepFailedPaymentAttempts bool `long:"keep-failed-payment-attempts" description:"Keeps persistent record of all failed payment attempts for successfully settled payments."`
 
 	StoreFinalHtlcResolutions bool `long:"store-final-htlc-resolutions" description:"Persistently store the final resolution of incoming htlcs."`
@@ -755,6 +761,7 @@ func DefaultConfig() Config {
 		ChannelCommitInterval:     defaultChannelCommitInterval,
 		PendingCommitInterval:     defaultPendingCommitInterval,
 		ChannelCommitBatchSize:    defaultChannelCommitBatchSize,
+		FwdHistoryDeleteBatchSize: defaultFwdHistoryDeleteBatchSize,
 		CoinSelectionStrategy:     defaultCoinSelectionStrategy,
 		KeepFailedPaymentAttempts: defaultKeepFailedPaymentAttempts,
 		RemoteSigner: &lncfg.RemoteSigner{
@@ -1722,6 +1729,19 @@ func ValidateConfig(cfg Config, interceptor signal.Interceptor, fileParser,
 			maxPendingCommitInterval)
 	}
 
+	// Warn and clamp fwd-history-delete-batch-size if it exceeds the DB
+	// layer maximum. The DB silently clamps anyway, but surfacing this at
+	// startup gives the operator immediate feedback that their configured
+	// value is not being honoured.
+	if cfg.FwdHistoryDeleteBatchSize > channeldb.MaxResponseEvents {
+		ltndLog.Warnf("fwd-history-delete-batch-size=%d exceeds "+
+			"maximum (%d), clamping to maximum",
+			cfg.FwdHistoryDeleteBatchSize,
+			channeldb.MaxResponseEvents)
+
+		cfg.FwdHistoryDeleteBatchSize = channeldb.MaxResponseEvents
+	}
+
 	if err := cfg.Gossip.Parse(); err != nil {
 		return nil, mkErr("error parsing gossip syncer: %v", err)
 	}
diff --git a/lnrpc/routerrpc/router_backend.go b/lnrpc/routerrpc/router_backend.go
index ca05606..3bb4d82 100644
--- a/lnrpc/routerrpc/router_backend.go
+++ b/lnrpc/routerrpc/router_backend.go
@@ -138,6 +138,12 @@ type RouterBackend struct {
 	// have before it can be deleted. If zero the handler defaults to 1
 	// hour.
 	MinForwardingHistoryAge time.Duration
+
+	// FwdHistoryDeleteBatchSize is the number of forwarding events deleted
+	// per database transaction. If zero the DB layer applies its own
+	// default (10 000). Exposed here so operators can tune the value via
+	// lnd.conf on resource-constrained nodes.
+	FwdHistoryDeleteBatchSize int
 }
 
 // ForwardingLogDB defines the interface for forwarding log database operations.
diff --git a/lnrpc/routerrpc/router_server.go b/lnrpc/routerrpc/router_server.go
index 40201cb..e3ac207 100644
--- a/lnrpc/routerrpc/router_server.go
+++ b/lnrpc/routerrpc/router_server.go
@@ -2036,9 +2036,7 @@ func (s *Server) DeleteForwardingHistory(ctx context.Context,
 			minAge, deleteBeforeTime, now)
 	}
 
-	// Default batch size is 10000, will be replaces by a config value in
-	// later commit.
-	batchSize := 10000
+	batchSize := s.cfg.RouterBackend.FwdHistoryDeleteBatchSize
 
 	log.Infof("DeleteForwardingHistory: deleting events at or before %v "+
 		"with batch size %d", deleteBeforeTime, batchSize)
diff --git a/rpcserver.go b/rpcserver.go
index 816635b..ded2bba 100644
--- a/rpcserver.go
+++ b/rpcserver.go
@@ -785,6 +785,9 @@ func (r *rpcServer) addDeps(ctx context.Context, s *server,
 		ShouldSetExpAccountability: func() bool {
 			return !s.cfg.ProtocolOptions.NoExpAccountability()
 		},
+		ForwardingLog:             s.miscDB.ForwardingLog(),
+		MinForwardingHistoryAge:   s.cfg.Dev.GetMinFwdHistoryAge(),
+		FwdHistoryDeleteBatchSize: s.cfg.FwdHistoryDeleteBatchSize,
 	}
 
 	genInvoiceFeatures := func() *lnwire.FeatureVector {
diff --git a/sample-lnd.conf b/sample-lnd.conf
index 6f3d849..f3a9c51 100644
--- a/sample-lnd.conf
+++ b/sample-lnd.conf
@@ -405,6 +405,11 @@
 ; a new commitment.
 ; channel-commit-batch-size=10
 
+; The number of forwarding events deleted per database transaction when running
+; deletefwdhistory. Lower this on resource-constrained nodes (e.g. Raspberry Pi)
+; to reduce lock contention. Maximum value is 50000.
+; fwd-history-delete-batch-size=10000
+
 ; Keeps persistent record of all failed payment attempts for successfully
 ; settled payments.
 ; keep-failed-payment-attempts=false