refactor(workload): plugin architecture wave + apps UI + volume scopes

Completes the workload-first refactor's plugin layer:

- internal/workload/plugin/ — Source/Trigger plugin contract,
  registry, types (Workload, DeploymentIntent, InboundEvent,
  PublicFace). Self-registering init() pattern + blank-import
  in cmd/server/main.go.
- Source plugins: image (blue-green with multi-face proxy routing),
  compose, static. Trigger plugins: registry, git, manual.
- internal/deployer/dispatch.go — DispatchPlugin/Teardown/Reconcile
  seam routing the legacy deployer through plugins.
- internal/api/workload_*.go — REST surface: workloads, env,
  volumes, chain (parent/children), promote-from. hooks.go
  serves /api/hooks/kinds/{kind}/schema for the wizard.
- internal/store: workload_env (encrypt-at-rest secrets) and
  workload_volumes tables, keyed on workload_id.
- cmd/server/static_backend.go — phantom-row adapter delegating
  the static source plugin to the legacy staticsite.Manager
  (deleted at hard cutover once the static inline port lands).
- web/src/routes/apps/ — /apps list + /apps/new wizard +
  /apps/[id] detail with kind-aware compose / image / static
  forms (Advanced JSON toggle), env panel, volumes panel,
  webhook panel, chain panel, manual deploy.

Volume scope generalization (v2 resolver):

- internal/volume.ResolveWorkloadPath (workload-keyed, sits
  next to legacy ResolvePath). Honors all VolumeScope values:
  absolute, ephemeral, instance, stage, project, project_named,
  named. internal/workload/plugin/source/image/image.go
  computeMounts wires settings + imageTag through. Coverage in
  internal/volume/resolver_test.go (portable Linux/Windows via
  t.TempDir).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/middleware.go

@@ -161,6 +161,24 @@ func jsonContentType(next http.Handler) http.Handler {
 	})
 }
 
+// deprecated marks responses with RFC-8594-style headers so API consumers
+// can detect that an endpoint is on its way out. The Workload-first
+// refactor is migrating away from /api/projects, /api/stages,
+// /api/static_sites, and /api/stacks toward /api/workloads; this signals
+// it to integrators without breaking them. Date is the operator-facing
+// sunset hint, not a hard switch.
+func deprecated(replacement string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Deprecation", "true")
+			if replacement != "" {
+				w.Header().Set("Link", `<`+replacement+`>; rel="successor-version"`)
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
 // rateLimitMiddleware wraps a handler with per-IP rate limiting using the
 // supplied limiter. Requests over the limit get 429.
 func rateLimitMiddleware(rl *rateLimiter) func(http.Handler) http.Handler {