laddr-import: tick the slug-reachability validation criterion now that read-api has landed

## Background

The [`laddr-import`](https://github.com/CodeForPhilly/codeforphilly-ng/blob/main/plans/laddr-import.md) plan (PR #24) left this Validation criterion unchecked:

> - [ ] All laddr slugs are accessible via `/projects/:slug` and `/members/:slug` after the import (verified via API test on a sample of 100 random records)

…with a Notes line claiming "those routes are owned by `read-api` and aren't built yet." That's not accurate at the time the PR was opened — `read-api` (PR #22) merged at 17:50 UTC; `laddr-import`'s PR opened at 22:29 UTC. The `/api/projects/:slug` and `/api/people/:slug` endpoints existed.

## What to do

Add an end-to-end test that:

1. Runs the importer against `apps/api/scripts/fixtures/laddr-fixture.sql` into a `createTestRepo()` + `createTestPrivateStore()`.
2. Boots `buildApp({ overrideEnv: { CFP_DATA_REPO_PATH: , ... } })`.
3. Picks 100 random imported records (or all of them if the fixture has fewer); for each, asserts `GET /api/projects/` and/or `GET /api/people/` returns the documented response shape.
4. Ticks the validation criterion in the plan.

Note: plans are frozen after merge, so the tick happens in this issue's PR via a small docs amendment under "Notes" rather than rewriting the criterion. Add a line like "Validation criterion 9 (slug reachability) closed out via Issue #N's tests; see [link]."

## Why bother

Closes the loop on a real validation gap. The import is one-shot — wrong slug → wrong URL forever. The test costs ~5min to write and pays for itself the first time it catches a slugify mistake.