M5-T09: add exposed_entities table and ExposableEntity provider framework

app/models/expose.py

@@ -0,0 +1,49 @@
+"""SQLAlchemy model for the exposed-entity toggle table.
+
+``ExposedEntityToggle`` stores the per-entity enable/disable state for the
+MQTT / HA Discovery expose framework.  The *catalog* of what *can* be
+exposed is computed dynamically by provider functions (see
+``app/integrations/expose.py``); this table only records which entries the
+user has explicitly enabled.
+
+Key design decisions
+--------------------
+- ``key`` is a stable string identifier derived from device uuid + metric key
+  (e.g. ``"modbus.<uuid>.voltage"``), so it does not drift if rows are
+  deleted and re-inserted.
+- Default state for an entity not yet in this table is **disabled** (false).
+  ``build_catalog`` in expose.py treats a missing row as ``enabled=False``.
+- Only one row per entity key (``key`` is UNIQUE).
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, Integer, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class ExposedEntityToggle(Base):
+    """Per-entity on/off switch for MQTT / HA Discovery publishing.
+
+    Rows are created on demand (first toggle); entities with no row are
+    treated as disabled.
+    """
+
+    __tablename__ = "exposed_entity_toggle"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    # Stable entity key, e.g. "modbus.<uuid>.voltage" or "modbus.<uuid>.online".
+    key: Mapped[str] = mapped_column(String(255), unique=True, nullable=False)
+
+    # Whether this entity should be published via MQTT / HA Discovery.
+    enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+
+    # Last time this row was created or modified.
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )