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>

backend/app/main.py

@@ -1,7 +1,7 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from app.api import bills, members, follows, dashboard, search, settings, admin, health, auth, notifications, notes
+from app.api import bills, members, follows, dashboard, search, settings, admin, health, auth, notifications, notes, collections, share
 from app.config import settings as config
 
 app = FastAPI(
@@ -29,3 +29,5 @@ app.include_router(admin.router, prefix="/api/admin", tags=["admin"])
 app.include_router(health.router, prefix="/api/health", tags=["health"])
 app.include_router(notifications.router, prefix="/api/notifications", tags=["notifications"])
 app.include_router(notes.router, prefix="/api/notes", tags=["notes"])
+app.include_router(collections.router, prefix="/api/collections", tags=["collections"])
+app.include_router(share.router, prefix="/api/share", tags=["share"])