Add a Russian README (README_ru.md), covered by the localized-README drift guard

The repo ships two localized READMEs, `README_zh.md` and `README_ja.md` (#317). Russian is not one of them.

**There is a concrete reason to want this one.** LibreDB Studio was posted to opennet.ru on 2026-09-06 (https://opennet.ru/66216-libredb) and the thread was a real technical discussion: installation without Docker, the security posture, the desktop wrapper, what the AI assistant sends where. Every reader who followed the link from there landed on an English README. A Russian one is the single highest-leverage translation the repo does not have.

**This is smaller than it looks.** `README.md` is 1002 lines; the two translations are 304 and 291. They are deliberate abridgements, not full translations - they carry the pitch, the engine list, the install table and the quick start, and they drop the deep sections. Copy that scope rather than translating the whole file.

**Fix.**

1. Add `README_ru.md`, modelled on `README_zh.md` for structure and length.
2. Add `Русский` to the language switcher near the top of `README.md`, `README_zh.md` and `README_ja.md` (they each list the other languages).
3. Add `"README_ru.md"` to `LOCALIZED` in `scripts/readme-check.mjs` (line 34), so the drift guard covers it.

**The guard decides two things for you, so review is objective.** `scripts/readme-check.mjs` runs inside the required `Lint, Typecheck and Build` check and asserts:

- **The engine name set is identical in all localized files and `README.md`.** A translation that omits an engine is wrong, and so is one that invents an engine. Do not translate engine names - `PostgreSQL` stays `PostgreSQL`.
- **Every command in a localized install table appears verbatim in `README.md`'s.** You may list fewer channels (both existing translations drop Chocolatey and the portable zip), but you may not paraphrase a command. A missing `sudo` or a dropped `brew trust` is exactly what this guard was written for after #317's review found both.

Run `bun run readme:check` before you push.

**Two notes specific to Russian, from that thread.** The audience reads the install table closely, so keep the native, non-container channels visible - deb/rpm, AppImage, snap, the standalone tarball, npm - rather than leading with Docker alone. And established Russian technical vocabulary beats literal translation: `тонкий/толстый клиент`, `образ`, `учётные данные`. Leave product names and CLI text in English.

**Please only take this if you read and write Russian well.** Raw machine-translation output is not what this is for, and a PR that reads as untouched machine output will be closed. Technical terms and product names are better left in English than translated approximately.

**Conflict note.** Step 2 touches one line in three files that every other language issue also touches (#594, #595, #596). If another translation lands first, rebase - it is a one-line conflict.

**Done when** `README_ru.md` exists, the switcher in all four READMEs lists it, `LOCALIZED` includes it, and `bun run readme:check` passes.

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Curated for Hacktoberfest 2026. Comment to claim it before you start so two people do not work on the same language. A PR must reference this issue; see CONTRIBUTING.md. Repo rules that apply: run `bun run test` (never bare `bun test`), and the 100% line-coverage gate must stay green. Local `bun run test` also needs the `helm` binary on PATH for unrelated chart tests - see #570 if it bites.

Start by comparing README_zh.md and README_ja.md with README.md to follow the localized scope and language-switcher structure. Update README.md, README_zh.md, README_ja.md, and scripts/readme-check.mjs, then run bun run readme:check; done means the Russian file, switcher entries, LOCALIZED entry, engine set, and install commands satisfy the guard.

Written by the indexing model from the issue text.