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/store/models.go

@@ -142,21 +142,26 @@ type DNSRecord struct {
 	UpdatedAt        string `json:"updated_at"`
 }
 
-// Instance represents a running (or stopped) container for a project stage.
-type Instance struct {
-	ID          string `json:"id"`
-	StageID     string `json:"stage_id"`
-	ProjectID   string `json:"project_id"`
-	ContainerID string `json:"container_id"`
-	ImageTag    string `json:"image_tag"`
-	Subdomain   string `json:"subdomain"`
-	NpmProxyID   int    `json:"npm_proxy_id"`
-	ProxyRouteID string `json:"proxy_route_id"`
-	Status       string `json:"status"` // running, stopped, failed, removing
+// ProxyRoute is a proxy-enabled container row joined with its project + stage
+// names, shaped for the Proxies page. Source is "instance" for project
+// containers and "static_site" for site rows — the names are historical
+// (the table itself was renamed to containers in the workload refactor).
+type ProxyRoute struct {
+	Source       string `json:"source"`
+	InstanceID   string `json:"instance_id"`
+	ProjectID    string `json:"project_id"`
+	ProjectName  string `json:"project_name"`
+	StageID      string `json:"stage_id"`
+	StageName    string `json:"stage_name"`
+	ImageTag     string `json:"image_tag"`
+	Subdomain    string `json:"subdomain"`
+	Domain       string `json:"domain"`
+	ContainerID  string `json:"container_id"`
 	Port         int    `json:"port"`
-	LastAliveAt string `json:"last_alive_at"`
-	CreatedAt   string `json:"created_at"`
-	UpdatedAt   string `json:"updated_at"`
+	ProxyRouteID string `json:"proxy_route_id"`
+	NpmProxyID   int    `json:"npm_proxy_id"`
+	Status       string `json:"status"`
+	CreatedAt    string `json:"created_at"`
 }
 
 // Deploy represents a deployment attempt.