fix: word-boundary variant filter; reattempt ALTCHA verify payload structure

- ytdlp.js: use \b word boundaries in all variant filter regexes so that
  partial matches like "nightcoreals" containing "nightcore" no longer
  falsely reject official videos; also fixes sped regression (!~= \bsped\b
  instead of the too-narrow "sped up" pattern)
- squid.js: restructure verify payload to mirror challenge response layout
  { parameters, solution: { number, derivedKey }, signature }

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/lib/ytdlp.js

@@ -28,19 +28,20 @@ function buildTitleKeywords(title) {
 
 function buildVariantFilter(title) {
   const t = (title || '').toLowerCase();
-  // !~= (?i) is yt-dlp's case-insensitive "does not match regex" — catches Remix/REMIX/remix etc.
+  // !~= (?i) = case-insensitive regex "does not match". Use \b word boundaries so that
+  // partial matches like "nightcoreals" containing "nightcore" don't falsely reject results.
   const RULES = [
-    { key: 'sped',         expr: 'title !~= (?i)sped up' },
-    { key: 'speedup',      expr: 'title !~= (?i)speed\\s*up' },
-    { key: 'slowed',       expr: 'title !~= (?i)slowed' },
-    { key: 'reverb',       expr: 'title !~= (?i)reverb' },
-    { key: 'nightcore',    expr: 'title !~= (?i)nightcore' },
-    { key: 'karaoke',      expr: 'title !~= (?i)karaoke' },
-    { key: 'parody',       expr: 'title !~= (?i)parody' },
-    { key: 'remix',        expr: 'title !~= (?i)remix' },
-    { key: 'cover',        expr: 'title !~= (?i)cover' },
-    { key: 'acoustic',     expr: 'title !~= (?i)acoustic' },
-    { key: 'instrumental', expr: 'title !~= (?i)instrumental' },
+    { key: 'sped',         expr: 'title !~= (?i)\\bsped\\b' },
+    { key: 'speedup',      expr: 'title !~= (?i)\\bspeed.{0,2}up\\b' },
+    { key: 'slowed',       expr: 'title !~= (?i)\\bslowed\\b' },
+    { key: 'reverb',       expr: 'title !~= (?i)\\breverb\\b' },
+    { key: 'nightcore',    expr: 'title !~= (?i)\\bnightcore\\b' },
+    { key: 'karaoke',      expr: 'title !~= (?i)\\bkaraoke\\b' },
+    { key: 'parody',       expr: 'title !~= (?i)\\bparody\\b' },
+    { key: 'remix',        expr: 'title !~= (?i)\\bremix\\b' },
+    { key: 'cover',        expr: 'title !~= (?i)\\bcover\\b' },
+    { key: 'acoustic',     expr: 'title !~= (?i)\\bacoustic\\b' },
+    { key: 'instrumental', expr: 'title !~= (?i)\\binstrumental\\b' },
   ];
   const parts = RULES.filter(r => !t.includes(r.key)).map(r => r.expr);
   return parts.length ? parts.join(' & ') : '';