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/src/routes/admin.js

@@ -20,8 +20,8 @@ function register(router) {
       // Try legacy SHA-256 verification for migration
       if (admin.password_salt && hashPasswordLegacy(password, admin.password_salt) === admin.password_hash) {
         // Auto-migrate to bcrypt
-        const newHash = await hashPassword(password)
-        await query('UPDATE admin_accounts SET password_hash = :password_hash, password_salt = NULL WHERE admin_id = :admin_id', { password_hash: newHash, admin_id: admin.admin_id })
+        const newHash = hashPassword(password)
+        await query('UPDATE admin_accounts SET password_hash = :password_hash, password_salt = :password_salt WHERE admin_id = :admin_id', { password_hash: newHash, password_salt: '', admin_id: admin.admin_id })
         matched = true
       }
     }