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/deployment/engine.go

@@ -20,6 +20,10 @@ type NodeRunner interface {
 	// ReconcileSpread distributes the service's replicas across every online,
 	// schedulable agent — replica i lands on agents[i % len].
 	ReconcileSpread(ctx context.Context, spec RuntimeSpec) (*RuntimeState, error)
+	// ReconcileAuto picks the least-loaded online agent carrying every tag in
+	// spec.PlacementTags and reconciles all replicas onto it. Re-resolved on
+	// every reconcile — the service follows the tag, not a stored node.
+	ReconcileAuto(ctx context.Context, spec RuntimeSpec) (*RuntimeState, error)
 	// ControlService fans a lifecycle action ("start"|"stop"|"restart"|"remove")
 	// out to every replica of the service on the agent.
 	ControlService(ctx context.Context, serviceID, agentID, action string) error