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/bills/BriefPanel.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState } from "react";
-import { ChevronDown, ChevronRight, RefreshCw } from "lucide-react";
+import { Check, ChevronDown, ChevronRight, RefreshCw, Share2 } from "lucide-react";
 import { BriefSchema } from "@/lib/types";
 import { AIBriefCard } from "@/components/bills/AIBriefCard";
 import { formatDate } from "@/lib/utils";
@@ -34,6 +34,14 @@ function typeBadge(briefType?: string) {
 export function BriefPanel({ briefs }: BriefPanelProps) {
   const [historyOpen, setHistoryOpen] = useState(false);
   const [expandedId, setExpandedId] = useState<number | null>(null);
+  const [copied, setCopied] = useState(false);
+
+  function copyShareLink(brief: BriefSchema) {
+    if (!brief.share_token) return;
+    navigator.clipboard.writeText(`${window.location.origin}/share/brief/${brief.share_token}`);
+    setCopied(true);
+    setTimeout(() => setCopied(false), 2000);
+  }
 
   if (!briefs || briefs.length === 0) {
     return <AIBriefCard brief={null} />;
@@ -57,6 +65,23 @@ export function BriefPanel({ briefs }: BriefPanelProps) {
         </div>
       )}
 
+      {/* Share button row */}
+      {latest.share_token && (
+        <div className="flex justify-end px-1">
+          <button
+            onClick={() => copyShareLink(latest)}
+            className="flex items-center gap-1.5 text-xs text-muted-foreground hover:text-foreground transition-colors"
+            title="Copy shareable link to this brief"
+          >
+            {copied ? (
+              <><Check className="w-3.5 h-3.5 text-green-500" /> Link copied!</>
+            ) : (
+              <><Share2 className="w-3.5 h-3.5" /> Share brief</>
+            )}
+          </button>
+        </div>
+      )}
+
       {/* Latest brief */}
       <AIBriefCard brief={latest} />