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/models/brief.py

@@ -1,4 +1,5 @@
 from sqlalchemy import Column, Integer, String, Text, ForeignKey, DateTime, Index
+from sqlalchemy.dialects import postgresql
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
@@ -21,6 +22,7 @@ class BillBrief(Base):
     llm_provider = Column(String(50))
     llm_model = Column(String(100))
     govinfo_url = Column(String, nullable=True)
+    share_token = Column(postgresql.UUID(as_uuid=False), nullable=True, server_default=func.gen_random_uuid())
     created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     bill = relationship("Bill", back_populates="briefs")