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/app/bills/[id]/page.tsx

@@ -12,6 +12,7 @@ import { ActionTimeline } from "@/components/bills/ActionTimeline";
 import { TrendChart } from "@/components/bills/TrendChart";
 import { NewsPanel } from "@/components/bills/NewsPanel";
 import { FollowButton } from "@/components/shared/FollowButton";
+import { CollectionPicker } from "@/components/bills/CollectionPicker";
 import { billLabel, chamberBadgeColor, congressLabel, formatDate, partyBadgeColor, cn } from "@/lib/utils";
 
 export default function BillDetailPage({ params }: { params: Promise<{ id: string }> }) {
@@ -109,7 +110,10 @@ export default function BillDetailPage({ params }: { params: Promise<{ id: strin
             )}
           </p>
         </div>
-        <FollowButton type="bill" value={bill.bill_id} supportsModes />
+        <div className="flex items-center gap-2 shrink-0">
+          <CollectionPicker billId={bill.bill_id} />
+          <FollowButton type="bill" value={bill.bill_id} supportsModes />
+        </div>
       </div>
 
       {/* Content grid */}