Audit: mass-budget reads/computes now emit audit-log entries for regulatory traceability

The mass budget computation (`calculerDevisMasse` / `computeMassBudget`) is a safety-critical calculation for flight authorization, yet the current read and compute paths do not generate audit log entries. This gap was identified during a review of PR #40, highlighting a regulatory traceability issue under Part-BOP.

Post-incident investigations may need to reconstruct the mass data shown to operators at flight-planning time. Without audit logs, this reconstruction is impossible, potentially compromising safety oversight. The proposed fix would emit audit entries on the vol detail page load or an explicit "show devis de masse" action, capturing key fields such as `volId`, `computed_charge_embarquee`, `max_payload`, `temperature_used`, and `surcharge_flag`. No personally identifiable information (PII) would be included, and the entries would be exploitant-scoped and role-aware.

A similar audit mechanism is being considered for performance chart updates on `Ballon`. While the severity is rated LOW—since this is not an immediate vulnerability—the issue underscores a critical gap in regulatory compliance and operational traceability. Addressing it now ensures that future investigations have the necessary data to verify safety-critical computations.