docs(onboarding): merchant intake checklist (EN + FR)

Practical field-by-field intake doc for the first merchants — maps
each question to its DB column so the call → admin UI is 1:1.
Includes a section on marketing-material reuse aimed at franchisees,
with written-authorization clauses for the future marketing module.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mkdocs.yml

@@ -339,6 +339,8 @@ nav:
       - Store Login Platform Detection: proposals/store-login-platform-detection.md
       - Test API Deps Auth Dependencies: proposals/test-api-deps-auth-dependencies.md
       - Post Soft-Delete Follow-ups: proposals/post-soft-delete-followups.md
+      - Merchant Intake Checklist: proposals/merchant-intake-checklist.md
+      - "Merchant Intake Checklist (FR)": proposals/merchant-intake-checklist-fr.md
 
     # --- Archive ---
     - Archive: