feat(scheduling): replica spread + resource-aware auto placement (#56)

services.spread distributes replicas across every online, schedulable
agent (replica i → agents[i mod n], deterministic). auto placement now
orders by real headroom — memory utilisation, cpu usage, then container
count — instead of count alone.

Lifecycle (start/stop/restart/delete) and GET /runtime are now driven
by container_instances inventory, so remote replicas are visible and
tear down wherever they run — covers spread, pins, and stale pin
leftovers. Spread and explicit pins are mutually exclusive.

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Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/src/lib/api-client.ts

@@ -86,6 +86,7 @@ export type ServiceEntity = {
   sleepIdleMinutes?: number;
   nodeId?: string;
   nodeName?: string;
+  spread?: boolean;
   publicUrl?: string;
 };
 
@@ -696,6 +697,7 @@ function normalizeService(service: RawService): ServiceEntity | null {
     sleepIdleMinutes: service.sleep_idle_minutes,
     nodeId: service.node_id,
     nodeName: service.node_name,
+    spread: service.spread,
     publicUrl: service.public_url,
   };
 }