fix(security): re-check inline CSS after template substitution

The fourth bypass found in this review, and the one no lexer fix
reaches: sanitizing runs on the stored body, but safeTemplateReplace
rewrites it afterwards, so the string that was validated is not the
string that is sent.

A conditional inside a style attribute can delete the very quoting that
made a url() inert:

  style="--x:x{{#if company_name}}'{{/if}};background:url(https://evil…)"

At write time the url() genuinely sits inside a CSS string and is
correctly left alone. Expanding the conditional for a recipient with no
company name removes both quotes and the background goes live —
confirmed end to end against the real functions.

The style-attribute pass now runs again on the substituted output.
Substitution cannot introduce a `"` (values are HTML-escaped), so the
attribute match still holds. body_css is not substituted, so the
<style> block cannot be rewritten after its check and needs nothing.

This is the case the removed newsletter pass had been covering. Rather
than reinstating a second definition of "disallowed", the one definition
now runs at both points where the content changes.

Refs #1264

backend/__tests__/services/newsletterService.sanitize.test.js

@@ -12,7 +12,9 @@ jest.mock('../../src/database/db', () => ({ db: jest.fn(), logActivity: jest.fn(
 
 const {
   sanitizeCampaignBody, sanitizeCampaignCss, MAX_BODY_BYTES,
+  sanitizeInlineStylesAfterSubstitution,
 } = require('../../src/services/newsletterService');
+const { safeTemplateReplace } = require('../../src/services/emailProcessor');
 
 describe('sanitizeCampaignBody', () => {
   it('returns empty string for empty input', () => {
@@ -188,3 +190,46 @@ describe('sanitizeCampaignCss', () => {
     expect(css).not.toContain('</style>');
   });
 });
+
+describe('inline CSS is re-checked after template substitution', () => {
+  // The stored body is sanitized, but safeTemplateReplace rewrites it
+  // afterwards — so the string that was validated is not the string that is
+  // sent. A conditional inside a style attribute can delete the quoting that
+  // made a url() inert, which no amount of lexer correctness can catch.
+  const payload =
+    `<p style="--x:x{{#if company_name}}'{{/if}};`
+    + `background:url(https://evil.example/p.gif);`
+    + `--y:x{{#if company_name}}'{{/if}}">hi</p>`;
+
+  it('neutralises a url() that substitution would activate', () => {
+    const stored = sanitizeCampaignBody(payload);
+    // Correctly left alone at write time: the url() really is inside a CSS
+    // string while the conditionals are still in place.
+    expect(stored).toContain('evil.example');
+
+    const substituted = safeTemplateReplace(stored, { company_name: '' }, { escapeHtml: true });
+    // Expansion removed the quotes, so without the recheck this ships live.
+    expect(substituted).toMatch(/background:url\(https:\/\/evil\.example/);
+
+    const rendered = sanitizeInlineStylesAfterSubstitution(substituted);
+    expect(rendered).not.toMatch(/url\(\s*['"]?https:\/\/evil\.example/);
+    expect(rendered).toContain('background:none');
+  });
+
+  it('leaves a body without style attributes untouched', () => {
+    const html = '<p>Hello {{first_name}}</p>';
+    expect(sanitizeInlineStylesAfterSubstitution(html)).toBe(html);
+  });
+
+  it('keeps legitimate inline styles through the recheck', () => {
+    const html = '<p style="color:red;font-size:14px">hi</p>';
+    const out = sanitizeInlineStylesAfterSubstitution(html);
+    expect(out).toContain('color:red');
+    expect(out).toContain('font-size:14px');
+  });
+
+  it('is safe on empty and nullish input', () => {
+    expect(sanitizeInlineStylesAfterSubstitution('')).toBe('');
+    expect(sanitizeInlineStylesAfterSubstitution(null)).toBeNull();
+  });
+});