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/api/share.py

@@ -0,0 +1,113 @@
+"""
+Public share router — no authentication required.
+Serves shareable read-only views for briefs and collections.
+"""
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+from app.database import get_db
+from app.models.bill import Bill, BillDocument
+from app.models.brief import BillBrief
+from app.models.collection import Collection, CollectionBill
+from app.schemas.schemas import (
+    BillSchema,
+    BriefSchema,
+    BriefShareResponse,
+    CollectionDetailSchema,
+)
+
+router = APIRouter()
+
+
+# ── Brief share ───────────────────────────────────────────────────────────────
+
+@router.get("/brief/{token}", response_model=BriefShareResponse)
+async def get_shared_brief(
+    token: str,
+    db: AsyncSession = Depends(get_db),
+):
+    result = await db.execute(
+        select(BillBrief)
+        .options(
+            selectinload(BillBrief.bill).selectinload(Bill.sponsor),
+            selectinload(BillBrief.bill).selectinload(Bill.briefs),
+            selectinload(BillBrief.bill).selectinload(Bill.trend_scores),
+        )
+        .where(BillBrief.share_token == token)
+    )
+    brief = result.scalar_one_or_none()
+    if not brief:
+        raise HTTPException(status_code=404, detail="Brief not found")
+
+    bill = brief.bill
+    bill_schema = BillSchema.model_validate(bill)
+    if bill.briefs:
+        bill_schema.latest_brief = bill.briefs[0]
+    if bill.trend_scores:
+        bill_schema.latest_trend = bill.trend_scores[0]
+
+    doc_result = await db.execute(
+        select(BillDocument.bill_id).where(BillDocument.bill_id == bill.bill_id).limit(1)
+    )
+    bill_schema.has_document = doc_result.scalar_one_or_none() is not None
+
+    return BriefShareResponse(
+        brief=BriefSchema.model_validate(brief),
+        bill=bill_schema,
+    )
+
+
+# ── Collection share ──────────────────────────────────────────────────────────
+
+@router.get("/collection/{token}", response_model=CollectionDetailSchema)
+async def get_shared_collection(
+    token: str,
+    db: AsyncSession = Depends(get_db),
+):
+    result = await db.execute(
+        select(Collection)
+        .options(
+            selectinload(Collection.collection_bills).selectinload(CollectionBill.bill).selectinload(Bill.briefs),
+            selectinload(Collection.collection_bills).selectinload(CollectionBill.bill).selectinload(Bill.trend_scores),
+            selectinload(Collection.collection_bills).selectinload(CollectionBill.bill).selectinload(Bill.sponsor),
+        )
+        .where(Collection.share_token == token)
+    )
+    collection = result.scalar_one_or_none()
+    if not collection:
+        raise HTTPException(status_code=404, detail="Collection not found")
+
+    cb_list = collection.collection_bills
+    bills = [cb.bill for cb in cb_list]
+    bill_ids = [b.bill_id for b in bills]
+
+    if bill_ids:
+        doc_result = await db.execute(
+            select(BillDocument.bill_id).where(BillDocument.bill_id.in_(bill_ids)).distinct()
+        )
+        bills_with_docs = {row[0] for row in doc_result}
+    else:
+        bills_with_docs = set()
+
+    bill_schemas = []
+    for bill in bills:
+        bs = BillSchema.model_validate(bill)
+        if bill.briefs:
+            bs.latest_brief = bill.briefs[0]
+        if bill.trend_scores:
+            bs.latest_trend = bill.trend_scores[0]
+        bs.has_document = bill.bill_id in bills_with_docs
+        bill_schemas.append(bs)
+
+    return CollectionDetailSchema(
+        id=collection.id,
+        name=collection.name,
+        slug=collection.slug,
+        is_public=collection.is_public,
+        share_token=collection.share_token,
+        bill_count=len(cb_list),
+        created_at=collection.created_at,
+        bills=bill_schemas,
+    )