fix: tolerate legacy NULL properties on pending devices

The pending_devices.properties column is added by an idempotent migration, so
existing rows have properties = NULL. PendingDeviceResponse typed it as a list,
so GET /scan/pending 500'd on any pre-existing device.

- Coerce NULL/non-list properties to [] in PendingDeviceResponse.
- Backfill existing NULL rows to '[]' in init_db migrations.
- Regression test: /scan/pending returns 200 with a legacy NULL-properties row.

ha-relevant: maybe

backend/app/db/database.py

@@ -117,6 +117,8 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE pending_devices ADD COLUMN discovery_source TEXT")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE pending_devices ADD COLUMN properties JSON")
+        with suppress(OperationalError):
+            await conn.exec_driver_sql("UPDATE pending_devices SET properties = '[]' WHERE properties IS NULL")
         with suppress(OperationalError):
             await conn.exec_driver_sql("ALTER TABLE scan_runs ADD COLUMN kind TEXT NOT NULL DEFAULT 'ip'")
         # --- Zigbee schema migrations (logged variant per CLAUDE.md feedback) ---

backend/app/schemas/scan.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 from typing import Any
 
-from pydantic import BaseModel
+from pydantic import BaseModel, field_validator
 
 
 class PendingDeviceResponse(BaseModel):
@@ -35,6 +35,12 @@ class PendingDeviceResponse(BaseModel):
     node_last_modified: datetime | None = None
     node_last_seen: datetime | None = None
 
+    @field_validator("properties", mode="before")
+    @classmethod
+    def _coerce_properties(cls, v: Any) -> list[Any]:
+        # Legacy rows (column added by migration) have properties = NULL.
+        return v if isinstance(v, list) else []
+
     model_config = {"from_attributes": True}