feat(dashboard): fingerprint discovery page — extract and group JA4/H2/headers from traffic

- GET /api/fingerprint-discovery: queries http_logs, groups by JA4, aggregates
  UA family, header presence rates (Sec-CH-UA, Sec-Fetch, Accept-Language,
  zstd, brotli, gzip, XFF), H2 data, TLS info, dict lookups
- /fingerprints page: KPIs, doughnut chart by family, stacked header bars,
  filterable/sortable profile table, expandable detail panel
- Promote button: push H2 fingerprints to browser_h2_signatures via existing
  POST /api/browser-signatures/entries endpoint
- Nav link: Découverte added after Navigateurs in sidebar

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

services/dashboard/backend/templates/base.html

@@ -163,6 +163,10 @@
                 <svg class="w-4 h-4 shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9.75 17L9 20l-1 1h8l-1-1-.75-3M3 13h18M5 17h14a2 2 0 002-2V5a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2z"/></svg>
                 <span class="nav-text">Navigateurs</span>
             </a>
+            <a href="/fingerprints" class="nav-item {% if active_page == 'fingerprints' %}active{% endif %}" title="Découverte de fingerprints (extraction DB)">
+                <svg class="w-4 h-4 shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z"/></svg>
+                <span class="nav-text">Découverte</span>
+            </a>
 
             <div class="nav-group-title">Investigation</div>
             <a href="/traffic" class="nav-item {% if active_page == 'traffic' %}active{% endif %}" title="Logs HTTP bruts">