Expose granular health indicators to StartOS

Add three new named health checks to the manifest alongside the existing
web check, each implemented as a fetch+parse of /api/health:

  ckpool      — is the stratum server alive? (miners can connect)
  bitcoin     — is Bitcoin Core RPC reachable? (blocks can be submitted)
  submit-gap  — have all block submissions been confirmed by bitcoind?

The web check retains its current role: calling /api/health which returns
503 when any critical subsystem is down, so StartOS marks the overall
service as degraded. The three new checks give operators a drill-down
view of which subsystem is the problem.

Each check returns "starting" for the first 20 s after launch so normal
boot sequencing does not trigger false alarms.

manifest.yaml

@@ -52,6 +52,18 @@ health-checks:
     name: "Web Dashboard"
     success-message: "Kamado dashboard is reachable."
     type: script
+  ckpool:
+    name: "Stratum Server"
+    success-message: "Stratum server is accepting connections."
+    type: script
+  bitcoin:
+    name: "Bitcoin Core RPC"
+    success-message: "Connected to Bitcoin Core."
+    type: script
+  submit-gap:
+    name: "Block Submission"
+    success-message: "All block submissions confirmed."
+    type: script
 
 config:
   get: