Add Document Discrepancy Finder tool

8-step NDJSON-streaming pipeline that compares two Barnevernet documents:
classifies each doc, extracts parties and timelines, cross-references both
for contradictions/deletions/additions, retrieves corpus legal context, and
synthesises a full discrepancy report with tabbed UI.

New files: DiscrepancyAgent.php, api/discrepancy.php, discrepancy.php,
discrepancy.js. Modified: FreeTier.php (cost=4), i18n.php (all 4 langs),
tool-svgs.php (DC icon), tools.css (dc-* component styles).

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

includes/FreeTier.php

@@ -25,6 +25,7 @@ final class FreeTier
         'advocate'      => 3,
         'deep-research' => 5,
         'transcribe'    => 2,  // flat rate; actual duration unknown upfront
+        'discrepancy'   => 4,  // 2 docs × 4 extraction steps + cross-ref + synthesis
     ];
 
     /** Credit cost for a given tool slug. Returns 1 for unknown tools. */