docs: добавить шаблон PR

Ссылается на чек-лист из CONTRIBUTING.md вместо дублирования, плюс
пункт про реальную проверку (не просто "тесты прошли" на словах) и
про отсутствие личных данных/ключей в дифе.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,20 @@
+## What does this change, and why?
+
+<!-- The "why" matters more than the "what" here — what problem does this
+     solve, or what couldn't you do before this change? If it fixes a bug,
+     what's the concrete repro? -->
+
+## Checklist
+
+- [ ] I actually ran `npm run build` (frontend) and/or `pytest` (backend) locally, not just assumed it would pass.
+- [ ] If this adds/changes tests, I ran them and watched them pass — not just wrote assertions that look right.
+- [ ] New/changed table or column → included an Alembic migration (see [CONTRIBUTING.md](../CONTRIBUTING.md#before-opening-a-pr)).
+- [ ] New user-facing text is in `frontend/src/lib/i18n.ts` for **both** `ru` and `en`, not hardcoded.
+- [ ] If this touches the UI, I checked it at a narrow (mobile) viewport, not just desktop.
+- [ ] No personal/financial data, credentials, or API keys anywhere in the diff — screenshots included, if any.
+- [ ] This PR does one thing. Unrelated fixes/refactors/formatting are split into separate PRs.
+
+## Screenshots / output
+
+<!-- For a UI change: before/after screenshots (desktop + mobile). For a
+     backend change: relevant command output (e.g. the test run). -->