docs: add visible slice local workflow

Document the migration/seed/API/web workflow in a root README, add an
API e2e smoke test that always checks /api/health and additionally
exercises the real DatabaseModule/seeded data plus the arrivesAt
validation rejection when DATABASE_URL is available, and add a
`test:e2e` root script.

Also fix `.env` loading so the documented root-level `.env` is actually
found: `main.ts` never loaded dotenv at all, and `data-source.ts`
loaded it relative to `process.cwd()`, which is `apps/api` (not the
repo root) whenever npm runs a `--workspace` script. Both now resolve
the repo-root `.env` explicitly. Also narrowed the CLI migrations glob
to numeric-prefixed files so it no longer tries to load the colocated
`*.migration.spec.ts` as a migration.

Verified end-to-end against a real PostgreSQL instance: migrate, seed
twice (idempotent), start the API, and exercise every documented route
including a rejected POST /api/travel body containing arrivesAt.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

apps/api/test/jest-e2e.json

@@ -5,5 +5,6 @@
   "testRegex": ".e2e-spec.ts$",
   "transform": {
     "^.+\\.(t|j)s$": "ts-jest"
-  }
+  },
+  "testTimeout": 15000
 }