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/router.py

@@ -4,8 +4,10 @@ from auth.dependencies import require_permission, require_roles
 from auth.models import Role
 from settings.models import MelodySettings, MelodySettingsUpdate
 from settings.public_features_models import PublicFeaturesSettings, PublicFeaturesSettingsUpdate
+from settings.log_retention_models import LogRetentionSettings, LogRetentionSettingsUpdate
 from settings import service
 from settings import public_features_service
+from settings import log_retention_service
 
 router = APIRouter(prefix="/api/settings", tags=["settings"])
 
@@ -40,3 +42,20 @@ async def update_public_features(
     _user: TokenPayload = Depends(require_roles(Role.sysadmin, Role.admin)),
 ):
     return public_features_service.update_public_features(body)
+
+
+# ── Log Retention Settings (sysadmin / admin only) ───────────────────────────
+
+@router.get("/log-retention", response_model=LogRetentionSettings)
+async def get_log_retention(
+    _user: TokenPayload = Depends(require_roles(Role.sysadmin, Role.admin)),
+):
+    return log_retention_service.get_log_retention()
+
+
+@router.put("/log-retention", response_model=LogRetentionSettings)
+async def update_log_retention(
+    body: LogRetentionSettingsUpdate,
+    _user: TokenPayload = Depends(require_roles(Role.sysadmin, Role.admin)),
+):
+    return log_retention_service.update_log_retention(body)