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/backend/sqlc/schema.sql

@@ -67,7 +67,8 @@ CREATE TABLE services (
     memory_reserve VARCHAR(20) NOT NULL DEFAULT '',
     static_build_cmd VARCHAR(255) NOT NULL DEFAULT '',
     static_dir VARCHAR(255) NOT NULL DEFAULT '',
-    node_id VARCHAR(255)
+    node_id VARCHAR(255),
+    spread BOOLEAN NOT NULL DEFAULT false
 );
 
 CREATE TABLE registries (