feat(scheduling): node tags + service placement_tags affinity (#58)

Operator-set node labels (node_agents.tags) gate which nodes a service
may schedule onto: services.placement_tags requires every listed tag.
auto and spread filter eligible agents by tags; tags are re-evaluated
each reconcile so retagging a node re-spreads replicas. Pin + tags and
local + tags rejected; empty array clears the requirement.

Surfaces: --placement-tags on services create/update, nodes update
--tags, MCP args, OpenAPI, node-detail tag editor, service-detail
placement-tags input.

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

app/backend/internal/api/runtime.go

@@ -94,6 +94,7 @@ func serviceRuntimeSpec(db *database.DB, service Service) (deployment.RuntimeSpe
 		RestartPolicy: service.RestartPolicy,
 		NodeID:        serviceNodeID(db, service.ID),
 		Spread:        serviceSpread(db, service.ID),
+		PlacementTags: servicePlacementTags(db, service.ID),
 	}
 	// Best effort: reuse the host port from the last live deployment so the
 	// public URL survives restarts. Column exists post-migration; older DBs