M0: implement operational core

Backend: SQLAlchemy models, Alembic migrations, requirements lockfile. Frontend: pinned deps, package-lock, vite-env types fix. Verified compose build/health, pytest, ruff clean.

backend/app/models/mixins.py

@@ -0,0 +1,24 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+
+class UUIDPrimaryKeyMixin:
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+
+
+class TimestampMixin:
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )