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/docker/client.go

@@ -9,17 +9,10 @@ import (
 
 // Labels applied to all containers managed by Tinyforge.
 //
-// Workload-shaped labels (LabelWorkloadID, LabelWorkloadKind, LabelRole,
-// LabelManaged) are the canonical set going forward and what the reconciler
-// queries by. The legacy project/stage/instance-id labels are still emitted
-// alongside them for back-compat with anything that selects on them
-// (operator runbooks, monitoring scrape rules, ad-hoc shell debugging) — they
-// will be removed once the migration soaks.
+// The legacy tinyforge.project / tinyforge.stage / tinyforge.instance-id
+// labels were removed in the workload refactor — the deployer now stamps
+// only the workload-shaped labels below at create time.
 const (
-	LabelProject    = "tinyforge.project"
-	LabelStage      = "tinyforge.stage"
-	LabelInstanceID = "tinyforge.instance-id"
-
 	LabelManaged      = "tinyforge.managed"       // present on every Tinyforge-managed container
 	LabelWorkloadID   = "tinyforge.workload.id"   // workload row primary key
 	LabelWorkloadKind = "tinyforge.workload.kind" // 'project' | 'stack' | 'site'