debug logs: new field, new queries, new APIs

_database/sql/migrations_squashed/debug_logs.sql

@@ -13,6 +13,9 @@ CREATE TABLE debug_logs (
   -- The type of debug log entry.
   debug_log_type VARCHAR(24) NOT NULL,
 
+  -- The severity level of the log entry (info, warn, error, debug, trace).
+  maybe_log_level VARCHAR(16) DEFAULT NULL,
+
   -- The user who created the log entry (if any).
   maybe_creator_user_token VARCHAR(32) DEFAULT NULL,
 
@@ -25,6 +28,7 @@ CREATE TABLE debug_logs (
   PRIMARY KEY (id),
   KEY index_event_token (event_token),
   KEY index_debug_log_type (debug_log_type),
+  KEY index_maybe_log_level (maybe_log_level),
   KEY index_maybe_creator_user_token (maybe_creator_user_token),
   KEY index_created_at (created_at)