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/stats/collector.go

@@ -195,38 +195,29 @@ type target struct {
 	OwnerID     string
 }
 
-// buildTargets fetches running instances and sites that have a container ID.
+// buildTargets fetches container rows that have a docker container_id bound.
+// Project containers and stack containers are surfaced as OwnerTypeInstance
+// (the stats sample owner_type is kept for back-compat with the persisted
+// schema and the dashboard's group-by semantics).
 func (c *Collector) buildTargets() []target {
 	var out []target
 
-	instances, err := c.store.ListAllInstances()
+	containers, err := c.store.ListContainers(store.ContainerFilter{})
 	if err != nil {
-		slog.Warn("stats collector: list instances", "error", err)
+		slog.Warn("stats collector: list containers", "error", err)
 	} else {
-		for _, inst := range instances {
-			if inst.ContainerID == "" {
+		for _, row := range containers {
+			if row.ContainerID == "" {
 				continue
 			}
-			out = append(out, target{
-				ContainerID: inst.ContainerID,
-				OwnerType:   OwnerTypeInstance,
-				OwnerID:     inst.ID,
-			})
-		}
-	}
-
-	sites, err := c.store.GetAllStaticSites()
-	if err != nil {
-		slog.Warn("stats collector: list sites", "error", err)
-	} else {
-		for _, site := range sites {
-			if site.ContainerID == "" {
-				continue
+			ownerType := OwnerTypeInstance
+			if row.WorkloadKind == string(store.WorkloadKindSite) {
+				ownerType = OwnerTypeSite
 			}
 			out = append(out, target{
-				ContainerID: site.ContainerID,
-				OwnerType:   OwnerTypeSite,
-				OwnerID:     site.ID,
+				ContainerID: row.ContainerID,
+				OwnerType:   ownerType,
+				OwnerID:     row.ID,
 			})
 		}
 	}