feat(settings): add log retention settings page

Sysadmin/admin-only settings page for configuring how long device log
history is kept before pruning. GET/PUT /api/settings/log-retention,
new LogRetentionSettings page, nav entry, and route.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/settings/log_retention_models.py

@@ -0,0 +1,10 @@
+from pydantic import BaseModel
+from typing import Optional
+
+
+class LogRetentionSettings(BaseModel):
+    days: int = 180
+
+
+class LogRetentionSettingsUpdate(BaseModel):
+    days: Optional[int] = None