Enrich found-block records with real coinbase reward

Adds RPC.GetBlock(hash, verbosity=2) and a CoinbaseReward
helper that sums the first tx's outputs. IngestBlockEvents
now does getblockhash -> getblock -> sum(vout) so
BlockRecord.RewardBT carries the actual BTC paid out on
solve instead of always being zero. Both RPC calls share a
single 5s deadline and are best-effort — bitcoind being
down just leaves the reward at zero.

api/internal/bitcoind/rpc.go

@@ -122,6 +122,50 @@ func (c *RPC) GetBlockHash(ctx context.Context, height int64) (string, error) {
 	return out, nil
 }
 
+// BlockVerbose2 is the subset of `getblock <hash> 2` we care about.
+// Verbosity 2 expands each tx into its full object so we can read the
+// coinbase output values without a second getrawtransaction call.
+type BlockVerbose2 struct {
+	Hash          string  `json:"hash"`
+	Height        int64   `json:"height"`
+	Confirmations int64   `json:"confirmations"`
+	Time          int64   `json:"time"`
+	Tx            []BlockTx `json:"tx"`
+}
+
+type BlockTx struct {
+	Txid string   `json:"txid"`
+	Vout []BlockVout `json:"vout"`
+}
+
+type BlockVout struct {
+	Value float64 `json:"value"`
+	N     int     `json:"n"`
+}
+
+// GetBlock returns a verbose-level-2 block decode for the given hash.
+func (c *RPC) GetBlock(ctx context.Context, hash string) (*BlockVerbose2, error) {
+	var out BlockVerbose2
+	if err := c.Call(ctx, "getblock", []any{hash, 2}, &out); err != nil {
+		return nil, err
+	}
+	return &out, nil
+}
+
+// CoinbaseReward sums all outputs of the first transaction in the block
+// (the coinbase). In solo mining this is the full subsidy + fees the
+// solving worker receives.
+func (b *BlockVerbose2) CoinbaseReward() float64 {
+	if len(b.Tx) == 0 {
+		return 0
+	}
+	var total float64
+	for _, vout := range b.Tx[0].Vout {
+		total += vout.Value
+	}
+	return total
+}
+
 // NetworkHashPS returns the network hashrate at the given block height.
 // `blocks` is a window (default 120). Pass -1 to use the default.
 func (c *RPC) GetNetworkHashPS(ctx context.Context, blocks, height int) (float64, error) {