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/shonar/db/migrate.py

@@ -0,0 +1,33 @@
+"""Programmatic schema migrations for the bundled desktop engine.
+
+Server deployments run ``alembic upgrade head`` in their deploy flow;
+the desktop app owns its SQLite file end-to-end, so the API applies
+migrations itself at startup (``SHONAR_AUTO_MIGRATE=1``).
+"""
+
+from __future__ import annotations
+
+import asyncio
+import logging
+from pathlib import Path
+
+from alembic import command
+from alembic.config import Config
+
+logger = logging.getLogger("shonar.migrate")
+
+# backend/ dir: migrations/ + alembic.ini live here relative to this file.
+_BACKEND_DIR = Path(__file__).resolve().parents[2]
+
+
+async def upgrade_head(database_url: str) -> None:
+    """Run 'alembic upgrade head' for `database_url` without blocking the loop."""
+
+    def _run() -> None:
+        cfg = Config(str(_BACKEND_DIR / "alembic.ini"))
+        cfg.set_main_option("script_location", str(_BACKEND_DIR / "migrations"))
+        cfg.set_main_option("sqlalchemy.url", database_url)
+        command.upgrade(cfg, "head")
+
+    await asyncio.to_thread(_run)
+    logger.info("database schema migrated to head")