docs: document backend runtime structure

backend/README.md

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+# dcalc backend
+
+## Run
+
+```bash
+uv sync
+python main.py
+```
+
+## Test
+
+```bash
+uv run pytest -v
+```
+
+## Quality
+
+```bash
+uv run ruff check .
+```
+
+## Runtime Entry
+
+- `main.py`: FastAPI application and uvicorn entry.
+- `api/routes/calc.py`: `/api/calc` HTTP route.
+- `services/calc_service.py`: calculation orchestration.
+- `core/basic/character/`: character calculation implementation.
+- `core/basic/equipment.py`: equipment data models and compatibility access.
+- `core/basic/equipment_registry.py`: equipment version registry and cache.
+- `database/connect.py`: SQLite and Redis connections.
+
+## Compatibility Rules
+
+- Do not change `/calc` request and response contract without frontend migration approval.
+- Do not change frontendNew result data shape during backend refactors.
+- Keep Skill and Forge business formulas behavior-compatible unless a dedicated formula task says otherwise.