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/state/blocks.go

@@ -35,11 +35,20 @@ func (a *Aggregator) IngestBlockEvents(ctx context.Context, events <-chan logmon
 				FoundAt: ev.SeenAt,
 				Source:  "logmon",
 			}
-			// Best-effort enrich with hash via bitcoind.
+			// Best-effort enrich with hash + coinbase reward via bitcoind.
+			// We look up the hash from height, then fetch the full block
+			// (verbosity 2) to sum the coinbase outputs. Both are fire-
+			// and-forget — if bitcoind is down we still record the block
+			// with whatever we have.
 			if a.RPC != nil {
 				lookupCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
 				if hash, err := a.RPC.GetBlockHash(lookupCtx, ev.Height); err == nil {
 					rec.Hash = hash
+					if blk, err := a.RPC.GetBlock(lookupCtx, hash); err == nil {
+						rec.RewardBT = blk.CoinbaseReward()
+					} else {
+						a.Log.Warn("bitcoind getblock failed", "hash", hash, "err", err)
+					}
 				} else {
 					a.Log.Warn("bitcoind getblockhash failed", "height", ev.Height, "err", err)
 				}