feat: collections, watchlists, and shareable links (v0.9.0)

Phase 3 completion — Personal Workflow feature set is now complete.

Collections / Watchlists:
- New tables: collections (UUID share_token, slug, public/private) and
  collection_bills (unique bill-per-collection constraint)
- Full CRUD API at /api/collections with bill add/remove endpoints
- Public share endpoint /api/collections/share/{token} (no auth)
- /collections list page with inline create form and delete
- /collections/[id] detail page: inline rename, public toggle,
  copy-share-link, bill search/add/remove
- CollectionPicker bookmark-icon popover on bill detail pages
- Collections nav link in sidebar (auth-required)

Shareable Brief Links:
- share_token UUID column on bill_briefs (backfilled on migration)
- Unified public share router at /api/share (brief + collection)
- /share/brief/[token] — minimal layout, full AIBriefCard, CTAs
- /share/collection/[token] — minimal layout, bill list, CTA
- Share2 button in BriefPanel header row, "Link copied!" flash

AuthGuard: /collections → AUTH_REQUIRED; /share prefix → NO_SHELL_PATHS

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

frontend/components/shared/Sidebar.tsx

@@ -3,6 +3,7 @@
 import Link from "next/link";
 import { usePathname, useRouter } from "next/navigation";
 import {
+  Bookmark,
   LayoutDashboard,
   FileText,
   Users,
@@ -25,6 +26,7 @@ const NAV = [
   { href: "/members", label: "Members", icon: Users, adminOnly: false, requiresAuth: false },
   { href: "/topics", label: "Topics", icon: Tags, adminOnly: false, requiresAuth: false },
   { href: "/following", label: "Following", icon: Heart, adminOnly: false, requiresAuth: true },
+  { href: "/collections", label: "Collections", icon: Bookmark, adminOnly: false, requiresAuth: true },
   { href: "/notifications", label: "Notifications", icon: Bell, adminOnly: false, requiresAuth: true },
   { href: "/settings", label: "Admin", icon: Settings, adminOnly: true, requiresAuth: false },
 ];