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/staticsite/manager.go

@@ -280,11 +280,9 @@ func (m *Manager) Deploy(ctx context.Context, siteID string, force bool) error {
 		NetworkID:    networkID,
 		Mounts:       mounts,
 		Labels: map[string]string{
-			"tinyforge.static-site":    site.ID,
+			"tinyforge.static-site":      site.ID,
 			"tinyforge.static-site-name": site.Name,
 		},
-		Project:      "static-site",
-		Stage:        site.Name,
 		WorkloadID:   m.resolveSiteWorkloadID(site.ID),
 		WorkloadKind: string(store.WorkloadKindSite),
 		Role:         "",
@@ -307,11 +305,12 @@ func (m *Manager) Deploy(ctx context.Context, siteID string, force bool) error {
 			NetworkID:    networkID,
 			Mounts:       mounts,
 			Labels: map[string]string{
-				"tinyforge.static-site":    site.ID,
+				"tinyforge.static-site":      site.ID,
 				"tinyforge.static-site-name": site.Name,
 			},
-			Project: "static-site",
-			Stage:   site.Name,
+			WorkloadID:   m.resolveSiteWorkloadID(site.ID),
+			WorkloadKind: string(store.WorkloadKindSite),
+			Role:         "",
 		})
 		if err != nil {
 			m.updateStatus(site.ID, "failed", latestSHA, fmt.Sprintf("create container: %v", err))