bitcoin: production-ready mainnet bitcoind for DRPC serving

Switch to bitcoin/bitcoin:31.0 with txindex and tuned RPC flags, drop the
esplora indexer sidecar, and add bitcoin-protocol health probes so fleet
status and wait-for-online work for bitcoin-family nodes.

Co-authored-by: Cursor <cursoragent@cursor.com>

compose_registry.json

@@ -1258,13 +1258,12 @@
     "compose_file": "bitcoin/bitcoind/bitcoin-mainnet-bitcoind-full",
     "features": [],
     "network": "bitcoin",
-    "node": "esplora",
+    "node": null,
     "relay": null,
     "stack": null,
     "type": "full",
     "volumes": [
-      "bitcoin-mainnet-bitcoind-full",
-      "bitcoin-mainnet-bitcoind-full_esplora"
+      "bitcoin-mainnet-bitcoind-full"
     ]
   },
   {
@@ -1273,13 +1272,12 @@
     "compose_file": "bitcoin/bitcoind/bitcoin-testnet-bitcoind-full",
     "features": [],
     "network": "bitcoin",
-    "node": "esplora",
+    "node": null,
     "relay": null,
     "stack": null,
     "type": "full",
     "volumes": [
-      "bitcoin-testnet-bitcoind-full",
-      "bitcoin-testnet-bitcoind-full_esplora"
+      "bitcoin-testnet-bitcoind-full"
     ]
   },
   {