feat: Member Effectiveness Score + Representation Alignment View (v0.9.9)

Member Effectiveness Score
- New BillCosponsor table (migration 0018) with per-bill co-sponsor
  party data required for the bipartisan multiplier
- bill_category column on Bill (substantive | commemorative | administrative)
  set by a cheap one-shot LLM call after each brief is generated
- effectiveness_score / percentile / tier columns on Member
- New bill_classifier.py worker with 5 tasks:
    classify_bill_category  — triggered from llm_processor after brief
    fetch_bill_cosponsors   — triggered from congress_poller on new bill
    calculate_effectiveness_scores — nightly at 5 AM UTC
    backfill_bill_categories / backfill_all_bill_cosponsors — one-time
- Scoring: distance-traveled pts × bipartisan (1.5×) × substance (0.1×
  for commemorative) × leadership (1.2× for committee chairs)
- Percentile normalised within (seniority tier × party) buckets
- Effectiveness card on member detail page with colour-coded bar
- Admin panel: 3 new backfill/calculate controls in Maintenance section

Representation Alignment View
- New GET /api/alignment endpoint: cross-references user's stanced bill
  follows (pocket_veto/pocket_boost) with followed members' vote positions
- Efficient bulk queries — no N+1 loops
- New /alignment page with ranked member list and alignment bars
- Alignment added to sidebar nav (auth-required)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -1,4 +1,4 @@
-from app.models.bill import Bill, BillAction, BillDocument
+from app.models.bill import Bill, BillAction, BillDocument, BillCosponsor
 from app.models.brief import BillBrief
 from app.models.collection import Collection, CollectionBill
 from app.models.follow import Follow
@@ -16,6 +16,7 @@ from app.models.vote import BillVote, MemberVotePosition
 __all__ = [
     "Bill",
     "BillAction",
+    "BillCosponsor",
     "BillDocument",
     "BillBrief",
     "BillNote",