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/share/brief/[token]/page.tsx

@@ -0,0 +1,78 @@
+"use client";
+
+import { use } from "react";
+import { useQuery } from "@tanstack/react-query";
+import Link from "next/link";
+import { ExternalLink, Landmark } from "lucide-react";
+import { shareAPI } from "@/lib/api";
+import { AIBriefCard } from "@/components/bills/AIBriefCard";
+import { billLabel } from "@/lib/utils";
+
+export default function SharedBriefPage({ params }: { params: Promise<{ token: string }> }) {
+  const { token } = use(params);
+
+  const { data, isLoading, isError } = useQuery({
+    queryKey: ["share-brief", token],
+    queryFn: () => shareAPI.getBrief(token),
+    retry: false,
+  });
+
+  return (
+    <div className="min-h-screen bg-background">
+      {/* Minimal header */}
+      <header className="border-b border-border bg-card px-6 py-3 flex items-center gap-2">
+        <Landmark className="w-5 h-5 text-primary" />
+        <Link href="/" className="font-semibold text-sm hover:opacity-70 transition-opacity">
+          PocketVeto
+        </Link>
+      </header>
+
+      <div className="max-w-2xl mx-auto px-4 py-8 space-y-6">
+        {isLoading && (
+          <div className="text-center py-20 text-muted-foreground text-sm">Loading…</div>
+        )}
+
+        {isError && (
+          <div className="text-center py-20">
+            <p className="text-muted-foreground">Brief not found or link is invalid.</p>
+          </div>
+        )}
+
+        {data && (
+          <>
+            {/* Bill label + title */}
+            <div>
+              <div className="flex items-center gap-2 mb-1">
+                <span className="font-mono text-sm font-semibold text-muted-foreground bg-muted px-2 py-0.5 rounded">
+                  {billLabel(data.bill.bill_type, data.bill.bill_number)}
+                </span>
+              </div>
+              <h1 className="text-xl font-bold leading-snug">
+                {data.bill.short_title || data.bill.title || "Untitled Bill"}
+              </h1>
+            </div>
+
+            {/* Full brief */}
+            <AIBriefCard brief={data.brief} />
+
+            {/* CTAs */}
+            <div className="flex flex-col sm:flex-row gap-3 pt-2">
+              <Link
+                href={`/bills/${data.bill.bill_id}`}
+                className="flex items-center gap-1.5 px-4 py-2 text-sm font-medium rounded-md border border-border hover:bg-accent transition-colors"
+              >
+                View full bill page <ExternalLink className="w-3.5 h-3.5" />
+              </Link>
+              <Link
+                href="/register"
+                className="flex items-center gap-1.5 px-4 py-2 text-sm font-medium rounded-md bg-primary text-primary-foreground hover:bg-primary/90 transition-colors"
+              >
+                Track this bill on PocketVeto →
+              </Link>
+            </div>
+          </>
+        )}
+      </div>
+    </div>
+  );
+}