Add experimental APIs: regional tailoring, one‑page briefing, and rapid issue response

[letskickk]

Motivation

• Provide lightweight, experimental tools to help candidate campaigns and small policy teams: A) 지역 맞춤 공약 보완, B) 후보 캠프용 1페이지 브리핑, D) 신속 이슈 대응 템플릿.
• These features are intended as advisory/templating (not central approval) and should be safe to expose under an experimental API namespace.

Description

• Added region profiling and tailoring logic (_region_profile, _build_tailor_tip) and new Pydantic models for responses: RegionalTailorItem, RegionalTailorResponse, OnePageBriefingResponse, RapidIssueRequest, and RapidIssueResponse in backend/main.py.
• Implemented three experimental endpoints: GET /api/experimental/candidates/{candidate_id}/regional-tailor, GET /api/experimental/candidates/{candidate_id}/briefing, and POST /api/experimental/issues/rapid-response which return structured recommendation/briefing/rapid-response templates derived from existing candidate data.
• Added focused unit tests tests/test_experimental_features.py that mock get_candidate_detail and validate output shapes and key strings.

Testing

• Ran PYTHONPATH=. pytest -q tests/test_experimental_features.py and it passed (3 passed).
• Attempted PYTHONPATH=. pytest -q tests/test_candidates_api.py to run existing candidate API tests and it failed in this environment (several failures related to API parameter handling and DB constraints in the test harness), which appear unrelated to the new experimental endpoints.
• The new endpoints are covered by the added tests and validated to return the expected structures in the test environment.

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Codex Task