Fix accelerator revenue impact always showing zero

The old approach compared coinbasevalue from two getblocktemplate
calls, but new mempool arrivals between calls masked the displacement
loss. Now uses a single template snapshot to find the marginal
(lowest fee-rate) transaction that would be displaced and reports
its fee as the revenue cost.

api/internal/bitcoind/rpc.go

@@ -273,11 +273,19 @@ func (c *RPC) GetNetworkHashPS(ctx context.Context, blocks, height int) (float64
 	return out, nil
 }
 
+// TemplateTx is one transaction inside a getblocktemplate result.
+type TemplateTx struct {
+	Txid   string `json:"txid"`
+	Fee    int64  `json:"fee"`    // satoshis
+	Weight int64  `json:"weight"` // weight units
+}
+
 // BlockTemplate is the subset of getblocktemplate we care about: the
 // coinbase value (subsidy + fees) and height of the next block.
 type BlockTemplate struct {
-	CoinbaseValue int64 `json:"coinbasevalue"` // satoshis
-	Height        int64 `json:"height"`
+	CoinbaseValue int64        `json:"coinbasevalue"` // satoshis
+	Height        int64        `json:"height"`
+	Transactions  []TemplateTx `json:"transactions"`
 }
 
 // GetBlockTemplate fetches the next block template with segwit rules.