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/schemas/schemas.py

@@ -140,6 +140,7 @@ class BriefSchema(BaseModel):
     llm_provider: Optional[str] = None
     llm_model: Optional[str] = None
     govinfo_url: Optional[str] = None
+    share_token: Optional[str] = None
     created_at: Optional[datetime] = None
 
     model_config = {"from_attributes": True}
@@ -295,3 +296,42 @@ class SettingsResponse(BaseModel):
     congress_poll_interval_minutes: int
     newsapi_enabled: bool
     pytrends_enabled: bool
+
+
+# ── Collections ────────────────────────────────────────────────────────────────
+
+class CollectionCreate(BaseModel):
+    name: str
+    is_public: bool = False
+
+    def validate_name(self) -> str:
+        name = self.name.strip()
+        if not 1 <= len(name) <= 100:
+            raise ValueError("name must be 1–100 characters")
+        return name
+
+
+class CollectionUpdate(BaseModel):
+    name: Optional[str] = None
+    is_public: Optional[bool] = None
+
+
+class CollectionSchema(BaseModel):
+    id: int
+    name: str
+    slug: str
+    is_public: bool
+    share_token: str
+    bill_count: int
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class CollectionDetailSchema(CollectionSchema):
+    bills: list[BillSchema]
+
+
+class BriefShareResponse(BaseModel):
+    brief: BriefSchema
+    bill: BillSchema