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/base.py

@@ -0,0 +1,50 @@
+"""Public SQLAlchemy model base."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import UTC, datetime
+
+from sqlalchemy import DateTime, TypeDecorator, Uuid
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
+
+
+class UTCDT(TypeDecorator):
+    """Timezone-aware datetime that survives SQLite round-trips.
+
+    Postgres returns aware values (impl is a no-op there); SQLite has no
+    tz info and hands back naive datetimes, which crash comparisons with
+    ``utcnow()``. Attach UTC on read whenever the driver lost it.
+    """
+
+    impl = DateTime(timezone=True)
+    cache_ok = True
+
+    def process_result_value(self, value, dialect):  # noqa: ANN001, ANN201
+        if value is not None and value.tzinfo is None:
+            return value.replace(tzinfo=UTC)
+        return value
+
+
+def utcnow() -> datetime:
+    return datetime.now(UTC)
+
+
+def new_uuid() -> uuid.UUID:
+    return uuid.uuid4()
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+class PublicIdMixin:
+    """Gives every table a UUID public id; integer PKs stay internal."""
+
+    id: Mapped[uuid.UUID] = mapped_column(Uuid, primary_key=True, default=new_uuid)
+    created_at: Mapped[datetime] = mapped_column(UTCDT(), default=utcnow, nullable=False)
+    # default= as well as onupdate=: SQLite enforces NOT NULL on insert
+    # (Postgres silently stored NULL for rows that never got an UPDATE).
+    updated_at: Mapped[datetime] = mapped_column(
+        UTCDT(), default=utcnow, onupdate=utcnow, nullable=False
+    )