Tighten fallback latency + alert UI on degraded states

Submit-first ordering. Patch 0004 now calls generator_submitblock
BEFORE writing the pending-block hex to disk. The happy path adds zero
disk I/O — we only dump when the primary returned false. The same
patch bounds generator_submitblock's "no live current_si" spin to
~3s instead of the original infinite loop, so a permanently-down
primary doesn't pin the stratifier; the bounded spin lets the caller
return false and lets local_block_submit dump for kamado-api to take
over.

Default grace lowered from 30s to 3s. With ckpool's bounded spin and
sub-second sweep cadence, the fallback now reacts within ~4s of a
failed primary submit — fast enough that the work is still relevant
for the current chain tip. The submitter's sweep poll dropped to 1s
to match.

UI HealthBanners. New top-of-page strip surfaces:
  * Fallback used (red banner, 24h after most recent event):
    "primary bitcoind didn't accept; backup X took over Y ago"
  * Submit gap (orange banner, only when no recent fallback):
    "N blocks attempted but unconfirmed — configure backups"
  * ZMQ stale (orange banner): no hashblock frame in 30+ minutes
Operators see degraded-but-not-fatal states without checking logs.

Startup readiness gate. main now waits up to 8s on agg.Ready() before
starting the HTTP server so the very first /api/snapshot doesn't show
all-zero state during the aggregator's first refresh. Capped so a
permanently-down bitcoind can't block startup; /healthz is honest
about the degraded state once we do start serving.

api/internal/blocksubmit/submit.go

@@ -71,7 +71,11 @@ func (s *Submitter) Run(ctx context.Context, poll time.Duration) {
 		return
 	}
 	if s.Grace == 0 {
-		s.Grace = 30 * time.Second
+		// Match the config default — short enough that a failed
+		// submit gets recovered while the work is still relevant,
+		// long enough that we don't spuriously fallback while the
+		// primary's RPC is mid-handshake.
+		s.Grace = 3 * time.Second
 	}
 	if s.MaxAge == 0 {
 		s.MaxAge = 24 * time.Hour