refactor: use actors for db configuration (#1604)

Co-authored-by: Claude <noreply@anthropic.com>

backend/resources/migrations/postgres/20260718000000_freeze_config.down.sql

@@ -0,0 +1,16 @@
+-- Recreate the standalone config table with the same schema it had before it was frozen
+CREATE TABLE app_config_variables
+(
+    key   VARCHAR(100) NOT NULL PRIMARY KEY,
+    value TEXT NOT NULL
+);
+
+-- Populate it from the frozen JSON document stored in the "kv" table
+-- json_each expands the JSON object back into one row per key/value pair.
+INSERT INTO app_config_variables (key, value)
+SELECT je.key, je.value
+FROM kv, json_each_text(kv."value"::json) AS je(key, value)
+WHERE kv."key" = 'config_migrated';
+
+-- Remove the frozen config from the "kv" table
+DELETE FROM kv WHERE "key" = 'config_migrated';