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/lib/types.ts

@@ -72,6 +72,7 @@ export interface BriefSchema {
   llm_provider?: string;
   llm_model?: string;
   govinfo_url?: string;
+  share_token?: string;
   created_at?: string;
 }
 
@@ -183,6 +184,20 @@ export interface NotificationSettings {
   timezone: string | null;  // IANA name, e.g. "America/New_York"
 }
 
+export interface Collection {
+  id: number;
+  name: string;
+  slug: string;
+  is_public: boolean;
+  share_token: string;
+  bill_count: number;
+  created_at: string;
+}
+
+export interface CollectionDetail extends Collection {
+  bills: Bill[];
+}
+
 export interface NotificationEvent {
   id: number;
   bill_id: string;