chore(backend): teach eslint the rest-sibling omission idiom

Adds varsIgnorePattern and ignoreRestSiblings to no-unused-vars, the config
recommendation left open when the lint backlog was cleared.

The "omit fields via rest spread" idiom is intentional and recurring --
adminEvents/helpers.js destructures password_hash and client_password_hash
purely to keep them out of `...rest` -- and without ignoreRestSiblings every
occurrence needs its own disable comment, which is noise that also suppresses
genuine findings on the same line. Removed the one such comment that now
exists; the explanatory comment above it stays, since the intent is not
obvious from the code.

Lint stays at 0 problems.

Refs testplan REPORT.md D1.

backend/.eslintrc.js

@@ -17,7 +17,15 @@ module.exports = {
     'linebreak-style': ['error', 'unix'],
     'quotes': ['error', 'single'],
     'semi': ['error', 'always'],
-    'no-unused-vars': ['error', { 'argsIgnorePattern': '^_' }],
+    // varsIgnorePattern + ignoreRestSiblings cover the "omit fields via rest
+    // spread" idiom (e.g. adminEvents/helpers.js pulling password hashes out
+    // of ...rest), which is intentional and would otherwise need a disable
+    // comment at every occurrence.
+    'no-unused-vars': ['error', {
+      'argsIgnorePattern': '^_',
+      'varsIgnorePattern': '^_',
+      'ignoreRestSiblings': true
+    }],
     'no-console': ['warn', { allow: ['warn', 'error'] }]
   }
 };

backend/src/routes/adminEvents/helpers.js

@@ -177,7 +177,6 @@ const mapEventForApi = (event) => {
     customer_email,
     customer_phone,
     // Bound only to exclude the secrets from `...rest` — never read.
-    // eslint-disable-next-line no-unused-vars -- rest-sibling omission
     password_hash: _ph, client_password_hash: _cph,
     ...rest
   } = event;