feat(voteballots): add WSP 97 confidence scoring integration (Slice 4/6) (#712)

Implement confidence scoring for VoteBallots funding summaries with
explicit WSP 97 confidence labels: VERIFIED_FACT, HIGH_CONFIDENCE_INFERENCE,
LOW_CONFIDENCE_INFERENCE, and UNKNOWN.

Key features:
- Direct FEC source with reference -> VERIFIED_FACT
- Missing source -> UNKNOWN or capped confidence
- Trail termination markers preserved and scored as UNKNOWN
- Human review triggers for high-risk claims:
  - Foreign funding allegation
  - Criminal accusation
  - Low confidence + high impact
  - Source contradiction
- Fail-closed error propagation from funding summary
- No quick answer or prose generation
- Source references preserved for provenance

Safety boundaries enforced:
- NO_FOREIGN_FUNDING_CLAIM_GENERATED (only flags for review)
- NO_DARK_MONEY_AS_VERIFIED_FACT
- NO_CANDIDATE_RECOMMENDATION
- NO_TARGETED_PERSUASION
- NO_QUICK_ANSWER_GENERATION
- HUMAN_REVIEW_FOR_HIGH_RISK_CLAIMS

Test coverage: 37 new tests (195 total), all passing.
Builds on: PR #707 (FEC), #709 (Entity), #710 (Funding Summary)

Worker-Lane: W6
Slice: VOTE_POC_CONFIDENCE_SCORING_INTEGRATION_PHASE1

Co-authored-by: Foundups Agent <dev@foundups.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: 3 people