Standalone Shonar Desktop: vendor portable sources + local engine; decouple from ~/Projects/Shonar

- shared/ = portable Android-origin sources vendored from deferred/desktop-server
  (app/build.gradle.kts srcDir repointed; PlaybackController.kt excluded as Android-only)
- backend/ = bundled-lite engine (SQLite + inline queue); .venv symlinked from the
  old checkout, PYTHONPATH pins THIS backend's code over any editable install
- repoRoot() resolves this project dir (env SHONAR_REPO still wins); desktop-dev.sh
  watches shared/ + backend/
- Verified: :app:compileKotlin + :app:test green (23 tests); engine boots on :8010,
  self-migrates, /healthz ok

backend/README.md

@@ -0,0 +1,16 @@
+# S.H.O.N.A.R. backend
+
+FastAPI + PostgreSQL backend for the S.H.O.N.A.R. Android app.
+
+See the repository root [README](../README.md) and `docs/` for full documentation.
+
+## Quick start (development)
+
+```bash
+cd backend
+uv venv .venv && uv pip install -e ".[dev]"
+cp ../.env.example .env        # then edit SHONAR_SECRET_KEY etc.
+uvicorn shonar.main:app --reload --port 8000
+```
+
+Tests: `pytest`  ·  Lint: `ruff check .`  ·  Migrations: `alembic upgrade head`