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/database/sqlcdb/models.go

@@ -177,6 +177,7 @@ type NodeAgent struct {
 	Metadata      pqtype.NullRawMessage `json:"metadata"`
 	AutoPrune     bool                  `json:"auto_prune"`
 	Schedulable   bool                  `json:"schedulable"`
+	Tags          json.RawMessage       `json:"tags"`
 }
 
 type Notification struct {
@@ -277,6 +278,7 @@ type Service struct {
 	StaticDir       string          `json:"static_dir"`
 	NodeID          sql.NullString  `json:"node_id"`
 	Spread          bool            `json:"spread"`
+	PlacementTags   json.RawMessage `json:"placement_tags"`
 }
 
 type ServiceDomain struct {