add-e-invoicing

# Add E-Invoicing

Domain-specific module for back-offices with a French B2B flow.
DGFiP reform: reception mandatory 2026-09-01, SME/micro-business issuance 2027-09-01.

Install mode (on an existing project):
  1. Write fingerprint into the project CONTEXT.md
  2. ADR: docs/adr/0001-e-invoicing-activation.md
  3. Add §11.bis to the PRD
  4. 5-ticket plan in todo.md

Non-negotiable rules:
  Terminology  : 'Plateforme Agréée' (not PDP, since July 2025)
  Immutability : issued invoice (status ≥ Déposée) = no UPDATE, no DELETE
                 only correction mechanism = Avoir (CreditNote)
  Numbering    : InvoiceSequence table (composite key tenant+series+year)
                 atomic transaction, no vanilla Prisma auto-increment
  Integrity    : SHA-256 hash of the PDF/A-3 in Invoice.hash at issuance
  Archiving    : S3 Object Lock COMPLIANCE mode, 10 years
                 GOVERNANCE forbidden (revocable = non-compliant)
  Abstraction  : no direct HTTP call to a PA outside an adapter
                 EInvoicingProvider interface
  Validation   : first invoice validated by the chartered accountant before prod

Audit mode (--audit):
  Detect drift vs the current templates
  Propose a patch plan
  Remind the age of last-regulatory-review (alert if >90 days)