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/src/main.ts

@@ -1,7 +1,15 @@
+import { config } from 'dotenv';
+import { resolve } from 'path';
 import { NestFactory } from '@nestjs/core';
 import { AppModule } from './app.module';
 import { configureApplication } from './app.config';
 
+// `dev:api` runs via the `apps/api` npm workspace, so process.cwd() is
+// `apps/api`, not the repo root where the documented `.env` lives. Resolve
+// it explicitly so `npm run dev:api` picks up `DATABASE_URL` after
+// `Copy-Item .env.example .env` as documented in the README.
+config({ path: resolve(__dirname, '..', '..', '..', '.env') });
+
 async function bootstrap() {
   const app = await NestFactory.create(AppModule);
   configureApplication(app);