Add authenticated POST /api/v1/jobs to enqueue registered job kinds

Addresses feature-review finding F1 on Feature 04 PR #30: a job can now
be enqueued over the API and its progressive result read back.

- jobs: Runner.Enqueue(kind, payload) rejects an unregistered kind with
  ErrUnknownKind and a payload that does not decode into the kind's
  payload type with ErrInvalidPayload, storing no job. Kind.Enqueue
  shares the same insert path.
- server: POST /api/v1/jobs, inside the session- and CSRF-guarded /api/v1
  group, takes {kind, payload?} and returns 202 {id}; unknown kind,
  bad payload or malformed body return 400 with the standard error shape.
- Tests drive POST -> GET partial result (handler held on a channel) ->
  GET succeeded through the production router, plus 400/401/403 cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/server.go

@@ -30,7 +30,7 @@ type RouteMounter interface {
 // only from a peer in trusted (see resolveOrigin). version is reported by
 // GET /health; auth mounts the /auth routes; sessions guards /api/v1 and
 // /media; settings serves /api/v1/settings/household; runner serves
-// /api/v1/jobs/{id}; assets is the web build
+// POST /api/v1/jobs and GET /api/v1/jobs/{id}; assets is the web build
 // served for every other GET. /health, /auth and the web build are public.
 // Requests to /api/v1 other than GET also need the session's CSRF token in CSRFHeader.
 func NewRouter(logger *slog.Logger, version string, trusted forwarded.Trusted, auth RouteMounter, sessions SessionLookup, settings *household.SettingsService, runner *jobs.Runner, assets fs.FS) (http.Handler, error) {
@@ -68,6 +68,7 @@ func NewRouter(logger *slog.Logger, version string, trusted forwarded.Trusted, a
 	apiV1.Get("/me", me(logger))
 	apiV1.Get("/settings/household", getHouseholdSettings(logger, settings))
 	apiV1.Patch("/settings/household", patchHouseholdSettings(logger, settings))
+	apiV1.Post("/jobs", postJob(logger, runner))
 	apiV1.Get("/jobs/{id}", getJob(logger, runner))
 	r.Route("/api", func(r chi.Router) {
 		r.MethodNotAllowed(methodNotAllowed)