Add Légifrance API integration for live legal text injection

Implements a two-layer approach to ensure legal texts are accurate and current:

Option A — runtime injection (legifrance_client.py):
- LegifranceClient: OAuth2 via PISTE (oauth.piste.gouv.fr), fetches consolidated
  article text via api.piste.gouv.fr/dila/legifrance/lf-engine-app
- Redis cache with 24h TTL (in-memory fallback if no Redis)
- build_filter_prompt() injects live texts into the prompt with explicit priority
  over static descriptions — model uses official Légifrance text, not training memory
- Graceful fallback to static prompt if API is unavailable — filtering never blocks

Option B — weekly sync script (scripts/check_legal_refs.py):
- Reads legal_refs.yaml (11 tracked articles, priority critical/high/medium)
- Fetches current text from Légifrance, diffs against stored baseline
- --init: resolves LEGIARTI IDs on first run
- --check: report only; --update: saves new texts to YAML
- Exit code 2 when changes detected (CI/n8n-compatible)

Supporting changes:
- legal_refs.yaml: tracked articles with code LEGITEXT IDs, priorities, rationale
- requirements.txt: add pyyaml>=6.0.2 and requests>=2.32.0
- .env.example: document PISTE_CLIENT_ID, PISTE_CLIENT_SECRET, LEGIFRANCE_CACHE_TTL
- transparence.tsx: document the Légifrance injection for public transparency
- Fixed portal URL: piste.gouv.fr (not piste.api.gouv.fr — verified June 2026)

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

artifacts/flask-api/requirements.txt

@@ -6,4 +6,6 @@ gunicorn>=23.0.0
 openai>=1.77.0
 psycopg2-binary>=2.9.10
 python-dotenv>=1.0.1
+pyyaml>=6.0.2
 redis>=5.0.0
+requests>=2.32.0