docs: add full project documentation

- Architecture: overview, 7 ADR, tech stack
- Principles: code-style, git-workflow, security
- API contracts: auth, profile, tests, admin endpoints
- Database schema: tables, relationships, indexes
- LLM strategy: prompts, fallback, validation, Qwen 2.5 14B
- Onboarding: setup, Docker, .env template
- Progress: roadmap, changelog
- Agents: context, backend instructions

Made-with: Cursor

progress/changelog.md

@@ -0,0 +1,12 @@
+# Changelog
+
+Формат: [дата] — краткое описание изменений.
+
+---
+
+## 2026-03-03
+
+- Создана структура документации `samreshu-docs`
+- Зафиксированы архитектурные решения (7 ADR)
+- Описана схема БД
+- Определён roadmap (MVP 0 → Phase 3)