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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app/backend/migrations_goose/20261112000000_placement_tags.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+-- Placement tags: operators label node_agents (tags jsonb array of strings),
+-- services require a tag set (placement_tags). "auto" and spread placement
+-- only consider online schedulable agents carrying every required tag.
+-- Explicit node_id pins bypass tag filtering — the pin is an operator override.
+ALTER TABLE services ADD COLUMN IF NOT EXISTS placement_tags jsonb NOT NULL DEFAULT '[]';
+ALTER TABLE node_agents ADD COLUMN IF NOT EXISTS tags jsonb NOT NULL DEFAULT '[]';
+
+-- +goose Down
+ALTER TABLE services DROP COLUMN IF EXISTS placement_tags;
+ALTER TABLE node_agents DROP COLUMN IF EXISTS tags;