Allow overriding Airtable base IDs via env vars

[ankit0504]

Issues resolved by this pull request

None — quality-of-life change for contributors who don't have access to the upstream ELA Airtable bases.

This PR is independent of the in-flight upgrade stack (#304, #305, #306). It targets `master` and can land in any order relative to those.

Prerequisites

• No install or build behavior changes.
• No `.env` changes required for anyone with access to the ELA production bases — the defaults are the existing hardcoded IDs, so existing setups are unchanged.
• Contributors without ELA base access can now duplicate the schema into their own workspace and set `REACT_APP_AIRTABLE_BASE` (and optionally `REACT_APP_AIRTABLE_CENSUS_BASE`) in `.env` to point the local dev build at it.

Review type

• FYI: small, additive change with no behavior change in the default case.
• Content/copy
• Needs feedback
• Functionality
• Code

What's in this PR

• `src/components/config/api.ts`: `AIRTABLE_BASE` and `AIRTABLE_CENSUS_BASE` now read from `process.env.REACT_APP_AIRTABLE_BASE` / `REACT_APP_AIRTABLE_CENSUS_BASE` with the existing hardcoded ELA IDs (`applPEl3BsnpuszQu`, `appjb6Qnp4lTNz7Gn`) as fallbacks. The defaults preserve current behavior.
• `sample.env`: document the two new optional vars with a comment explaining when a contributor would want to set them.

Motivation

The project README already warns: "Your setup will need to match ours, or you'll want to edit the code to your needs." This removes the "edit the code" friction — contributors without ELA base access can now run the app locally against their own duplicated base without making a local-only code change to `api.ts` that has to be stashed before every push.

Verification done locally

• `yarn build` — passes (CRA production build unchanged).
• With `REACT_APP_AIRTABLE_BASE` set in a local `.env`, runtime Airtable requests now target the override base; with it unset they continue to target the ELA base.

What to review

• `AIRTABLE_BASE` and `AIRTABLE_CENSUS_BASE` still resolve to the ELA IDs when the new env vars are unset.
• No other code reaches the `applPEl3BsnpuszQu` / `appjb6Qnp4lTNz7Gn` literals directly (confirmed via grep — only `api.ts` referenced them).
• `sample.env` comment is accurate and useful.