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/migrations/script.py.mako

@@ -0,0 +1,26 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+${imports if imports else ""}
+revision: str = ${repr(up_revision)}
+down_revision: str | None = ${repr(down_revision)}
+branch_labels: str | Sequence[str] | None = ${repr(branch_labels)}
+depends_on: str | Sequence[str] | None = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    ${downgrades if downgrades else "pass"}