feat: proposal intake pipeline with AI-powered generation on /proposals/new

Add full proposal system: DB schema (proposals + proposal_gaps tables),
CLI `ietf intake` command, and web UI with Quick Generate on /proposals/new.
The new page merges AI intake (paste URL/text → Haiku generates multiple
proposals auto-linked to gaps) with manual form entry. Generated proposals
are clickable cards that fill the editor below for refinement.

Uses claude_model_cheap (Haiku) for cost-efficient web intake. Includes
CaML-inspired draft proposals from arXiv:2503.18813 analysis.

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

src/webui/templates/base.html

@@ -132,6 +132,10 @@
                 <svg class="w-4 h-4 opacity-60" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-2.5L13.732 4.5c-.77-.833-2.694-.833-3.464 0L3.34 16.5c-.77.833.192 2.5 1.732 2.5z"/></svg>
                 Gap Explorer
             </a>
+            <a href="/proposals" class="sidebar-link flex items-center gap-3 px-5 py-2.5 text-sm text-slate-300 {{ 'active' if active_page == 'proposals' }}">
+                <svg class="w-4 h-4 opacity-60" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M11 5H6a2 2 0 00-2 2v11a2 2 0 002 2h11a2 2 0 002-2v-5m-1.414-9.414a2 2 0 112.828 2.828L11.828 15H9v-2.828l8.586-8.586z"/></svg>
+                Proposals
+            </a>
             {% endif %}
             <a href="/timeline" class="sidebar-link flex items-center gap-3 px-5 py-2.5 text-sm text-slate-300 {{ 'active' if active_page == 'timeline' }}">
                 <svg class="w-4 h-4 opacity-60" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 8v4l3 3m6-3a9 9 0 11-18 0 9 9 0 0118 0z"/></svg>