Service: job event stream, async in-memory backend, preview routes

Ported from PR #15 (packages-side surface only; the fork's dev UI page is
intentionally dropped - the API surface is the product):
- job_events table + repository, worker emits stage transitions, additive
  GET /v1/clones/:id/events polling route. The fork's hand-written
  migration was never registered in Drizzle's journal (it silently never
  applied); regenerated properly via drizzle-kit from schema.ts with the
  (job_id, seq) index declared, and verified against a live Postgres
- In-memory backend moves to the documented enqueue-202 + poll contract
  with single-flight BUSY guard; POST honors backend httpStatus
- app-preview static serving routes; the preview BUILD half is deferred
  until the compiler exports buildApp/DEFAULT_HARNESS_DIR - routes 404
  cleanly until then
- preview option threaded through core types; cache key intentionally
  unchanged while the flag is inert

All workspaces typecheck; api 18/18, core 13/13, worker 1/1 pass;
migration verified applying (table + index + FK) on postgres:16.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/db/migrations/0002_productive_wallflower.sql

@@ -0,0 +1,10 @@
+CREATE TABLE "job_events" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"job_id" uuid NOT NULL,
+	"seq" integer NOT NULL,
+	"payload" jsonb NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+ALTER TABLE "job_events" ADD CONSTRAINT "job_events_job_id_jobs_id_fk" FOREIGN KEY ("job_id") REFERENCES "public"."jobs"("id") ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+CREATE INDEX "job_events_job_id_seq_idx" ON "job_events" USING btree ("job_id","seq");