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/meta/_journal.json

@@ -15,6 +15,13 @@
       "when": 1782790200000,
       "tag": "0001_signup_tokens",
       "breakpoints": true
+    },
+    {
+      "idx": 2,
+      "version": "7",
+      "when": 1783202414928,
+      "tag": "0002_productive_wallflower",
+      "breakpoints": true
     }
   ]
-}
+}