fix: resolve password migration crash and schema constraints

- Login bcrypt migration: write password_salt = '' instead of NULL,
  which violated the NOT NULL constraint and caused 500 errors
- Schema: widen password_hash from CHAR(64) to VARCHAR(100) for bcrypt
  compatibility, add DEFAULT '' to password_salt
- init-db: add ALTER TABLE migrations for existing databases
- app.js: enrich error logging with method, path, SQL error details

server/sql/schema.sql

@@ -128,8 +128,8 @@ CREATE TABLE IF NOT EXISTS operation_logs (
 CREATE TABLE IF NOT EXISTS admin_accounts (
   admin_id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
   username VARCHAR(64) NOT NULL,
-  password_hash CHAR(64) NOT NULL,
-  password_salt CHAR(32) NOT NULL,
+  password_hash VARCHAR(100) NOT NULL,
+  password_salt VARCHAR(32) NOT NULL DEFAULT '',
   real_name VARCHAR(100) NOT NULL DEFAULT '',
   role VARCHAR(32) NOT NULL DEFAULT 'admin',
   status TINYINT NOT NULL DEFAULT 1,