Expose Block Broadcast Fallback config + plumb env through

New advanced.block-broadcast-fallback union:
  * enabled (default: false)
  * grace-seconds (default: 30, range 10-600)
  * backup-rpcs: list of full bitcoind RPC URLs, with inline
    credentials supported (https://user:pass@host:port/). Up to 10.

docker_entrypoint reads the list with yq, joins with newlines, and
exports BACKUP_RPC_URLS plus PENDING_BLOCKS_DIR=<logdir>/pending-blocks
and PENDING_BLOCKS_GRACE so kamado-api's blocksubmit watcher activates.
If the user enables the fallback but provides no URLs, we log a
warning and disable rather than spam pending-block warnings.

The companion ckpool patch (KamadoPool repo, 0004-dump-pending-block-
for-fallback) writes raw block hex to PENDING_BLOCKS_DIR before each
submit attempt and unlinks on success. The watcher only fires if
ckpool's primary submit fails AND the operator configured backup RPCs.

docker_entrypoint.sh

@@ -63,6 +63,30 @@ else
 fi
 export MEMPOOL_BASE_URL
 
+# Block-broadcast fallback: if the user enabled the safety-net
+# broadcaster AND provided at least one backup RPC URL, point
+# kamado-api at the patched-ckpool sidecar dir and feed the URL list
+# through. Empty PENDING_BLOCKS_DIR disables the watcher entirely.
+FALLBACK_ENABLED=$(q '.advanced.block-broadcast-fallback.enabled // false')
+if [[ "${FALLBACK_ENABLED}" == "true" ]]; then
+    # Newline-joined list — matches kamado-api's parser. q strips
+    # quotes via -r, and yq emits one entry per line for [] expansion.
+    BACKUP_RPC_URLS=$(q '.advanced.block-broadcast-fallback.backup-rpcs[]' 2>/dev/null || true)
+    if [[ -z "${BACKUP_RPC_URLS}" ]]; then
+        echo "Block-broadcast fallback enabled but no backup RPC URLs listed — disabling." >&2
+        export PENDING_BLOCKS_DIR=""
+        export BACKUP_RPC_URLS=""
+    else
+        # ckpool's patch writes hex files to <logdir>/pending-blocks.
+        export PENDING_BLOCKS_DIR="${LOGDIR}/pending-blocks"
+        export BACKUP_RPC_URLS
+        export PENDING_BLOCKS_GRACE=$(q '.advanced.block-broadcast-fallback.grace-seconds // 30')s
+    fi
+else
+    export PENDING_BLOCKS_DIR=""
+    export BACKUP_RPC_URLS=""
+fi
+
 # CKPool-solo uses the worker's stratum username as the payout
 # address and refuses to authenticate workers whose username is not
 # a valid address on the active network. The conf `btcaddress` is

scripts/procedures/getConfig.ts

@@ -213,6 +213,48 @@ export const getConfig: T.ExpectedExports.getConfig = compat.getConfig({
           },
         },
       },
+      "block-broadcast-fallback": {
+        "type": "object",
+        "name": "Block Broadcast Fallback",
+        "description":
+          "Safety net for the moment a share meets network difficulty. ckpool always submits the block to its primary bitcoind first; if that fails (bitcoind crashed, RPC saturated, peering broken), kamado-api re-broadcasts the raw block to one or more backup RPC URLs you list here. A block is only re-submitted if it hasn't been accepted within the grace window — well-behaved primaries are never disturbed.",
+        "spec": {
+          "enabled": {
+            "type": "boolean",
+            "name": "Enable fallback broadcaster",
+            "description":
+              "Off by default. Turning on requires at least one Backup RPC URL — without one we'd just log warnings about pending blocks we can't help with.",
+            "default": false,
+          },
+          "grace-seconds": {
+            "type": "number",
+            "name": "Grace period (seconds)",
+            "description":
+              "How long a block waits before kamado-api considers ckpool's submission attempt failed and broadcasts via fallback. 30s gives ckpool's own retry loop room to succeed against the primary.",
+            "nullable": false,
+            "range": "[10,600]",
+            "integral": true,
+            "units": "seconds",
+            "default": 30,
+          },
+          "backup-rpcs": {
+            "type": "list",
+            "name": "Backup Bitcoin RPC URLs",
+            "description":
+              "Each entry is a full bitcoind RPC URL, optionally with credentials inline (https://user:pass@host:8332/). Used in order. Anything that speaks the standard JSON-RPC submitblock method works — a second self-hosted bitcoind, an authenticated public RPC service (Quicknode, GetBlock.io, Allnodes, etc.), or a peer node you trust. Leaving this empty disables the broadcaster.",
+            "subtype": "string",
+            "spec": {
+              "pattern": "^https?://[^\\s]+$",
+              "pattern-description":
+                "Must be an http:// or https:// URL with no whitespace.",
+              "masked": false,
+              "copyable": false,
+            },
+            "default": [],
+            "range": "[0,10]",
+          },
+        },
+      },
     },
   },
 });