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/deployer/rollback.go

@@ -32,24 +32,25 @@ func (d *Deployer) rollback(ctx context.Context, deployID string, containerID st
 		}
 	}
 
-	// Clean up DNS record if the instance had a subdomain.
+	// Clean up DNS record if the container had a subdomain. instanceID is
+	// the container row ID (same UUID either way) — read from containers.
 	if instanceID != "" {
-		inst, err := d.store.GetInstanceByID(instanceID)
-		if err == nil && inst.Subdomain != "" {
+		c, err := d.store.GetContainerByID(instanceID)
+		if err == nil && c.Subdomain != "" {
 			settings, settingsErr := d.store.GetSettings()
 			if settingsErr != nil {
 				slog.Warn("rollback: failed to get settings for DNS cleanup", "error", settingsErr)
 			} else if settings.Domain != "" {
-				fqdn := inst.Subdomain + "." + settings.Domain
+				fqdn := c.Subdomain + "." + settings.Domain
 				d.removeDNS(ctx, fqdn, deployID)
 			}
 		}
 	}
 
-	// Update instance status to failed if it was created.
+	// Mark the container row as failed if it was created.
 	if instanceID != "" {
-		if err := d.store.UpdateInstanceStatus(instanceID, "failed"); err != nil {
-			slog.Warn("rollback: update instance status", "instance_id", instanceID, "error", err)
+		if err := d.store.UpdateContainerState(instanceID, "failed"); err != nil {
+			slog.Warn("rollback: update container state", "id", instanceID, "error", err)
 		}
 	}