refactor(workload): extract Instance entirely; Container is canonical

End-to-end extraction of the Instance concept. After this commit:

  * internal/store/instances.go — DELETED
  * internal/store/models.go — Instance struct gone, ProxyRoute moved here
  * containers table is the single source of truth for project/stack/site
    container state. instances table is dropped via DROP TABLE migration
    (idempotent; re-runnable on every boot).
  * Legacy tinyforge.project / tinyforge.stage / tinyforge.instance-id
    Docker labels are no longer emitted; only tinyforge.workload.{id,kind},
    tinyforge.role, and tinyforge.managed are stamped on new containers.

Backend rewrites:
  - internal/deployer:        executeDeploy + blueGreenDeploy + rollback +
                              promote use store.Container natively. New
                              removeContainer() replaces removeInstance().
                              enforceMaxInstances reads via
                              ListContainersByStageID.
  - internal/reconciler:      legacy tinyforge.instance-id dispatch removed;
                              upsertByWorkloadLabel now finds existing rows
                              by docker container ID first and falls back to
                              the deterministic workloadID:role key.
  - internal/stale/scanner:   Scan + new FindStaleContainers walk the
                              containers table; emit StaleContainer JSON.
  - internal/stats/collector: ListContainers replaces ListAllInstances.
  - internal/webhook/handler: workload-secret lookup tried first; falls back
                              to project / static_site secret column.
  - internal/api: instances.go, stale.go, stats.go, stats_history.go,
                  projects.go, settings.go, docker.go, dns.go all read /
                  write through Container.

Docker layer:
  - ManagedContainer exposes WorkloadID/Kind/Role from the canonical labels.
  - ListContainers filters by tinyforge.managed=true.
  - Network creation uses LabelManaged instead of LabelProject.

Frontend:
  - Instance type is now a Container alias; .status → .state,
    .last_alive_at → .last_seen_at.
  - InstanceCard takes stageId as a prop (no longer derived from Instance).
  - StaleContainer JSON shape rewritten: { container, workload_name, role,
    days_stale }. StaleContainerCard + /containers/stale page updated.
  - ProjectCard / homepage / SystemHealthCard filter by .state.

The migration loop now tolerates "no such table" alongside "duplicate
column" / "already exists" so obsolete ALTER TABLE entries targeting the
dropped instances table no-op cleanly on first boot.

Tests: store + deployer + reconciler + webhook + staticsite + notify all
still pass. Frontend svelte-check: zero errors.

internal/api/dns.go

@@ -204,9 +204,9 @@ func (s *Server) buildConsumerNameMap() map[string]string {
 	for _, p := range projects {
 		stages, _ := s.store.GetStagesByProjectID(p.ID)
 		for _, st := range stages {
-			instances, _ := s.store.GetInstancesByStageID(st.ID)
-			for _, inst := range instances {
-				names["instance:"+inst.ID] = p.Name + "/" + st.Name + ":" + inst.ImageTag
+			rows, _ := s.store.ListContainersByStageID(st.ID)
+			for _, c := range rows {
+				names["instance:"+c.ID] = p.Name + "/" + st.Name + ":" + c.ImageTag
 			}
 		}
 	}
@@ -362,15 +362,15 @@ func (s *Server) computeExpectedFQDNs(settings store.Settings) (map[string]strin
 			if !st.EnableProxy {
 				continue
 			}
-			instances, err := s.store.GetInstancesByStageID(st.ID)
+			rows, err := s.store.ListContainersByStageID(st.ID)
 			if err != nil {
-				slog.Warn("dns: failed to get instances", "stage_id", st.ID, "error", err)
+				slog.Warn("dns: failed to get containers", "stage_id", st.ID, "error", err)
 				continue
 			}
-			for _, inst := range instances {
-				if inst.NpmProxyID > 0 && inst.Subdomain != "" && inst.Status == "running" {
-					fqdn := inst.Subdomain + "." + settings.Domain
-					expected[fqdn] = "instance:" + inst.ID
+			for _, c := range rows {
+				if c.NpmProxyID > 0 && c.Subdomain != "" && c.State == "running" {
+					fqdn := c.Subdomain + "." + settings.Domain
+					expected[fqdn] = "instance:" + c.ID
 				}
 			}
 		}