feat: marjă internă per mechanic — hidden margin on labor

Client sees only Total. Salary is calculated from salary_base = client_price
× (1 − margin/100). Margin never appears in customer-facing surfaces (PDF,
tracking JSON, portal).

Terminology: "marjă internă" — internal profit margin. NOT VAT/TVA. Never
called NDS/TVA anywhere in the code to avoid confusion with real Moldova
tax reporting (Doc 19/1C integration).

== Configuration ==

Fallback chain (in MarginResolver::resolve):
  1. WorkOrder.override_margin_pct — per-Fișă for special contracts/VIP
  2. User.internal_margin_pct — per-mechanic (main setting)
  3. Company.settings.default_internal_margin_pct — tenant default
  4. 0.0 — no margin

Example (mechanic Andrei with 20% margin):
  User enters price_per_hour = 250 for 1h diagnosis
  → total = 250 (what client sees, goes into PDF)
  → salary_base = 250 × 0.80 = 200 (what mechanic gets salaried on)
  → applied_margin_pct = 20 (frozen)

If admin later changes Andrei's margin to 40%, the row's salary_base does
NOT change — history is immutable. Only new rows use the new margin.
Solves the retroactive-recompute problem for closed payroll periods.

== salary_base freeze semantics ==

wo_works gains 2 columns:
  salary_base decimal(10,2) nullable
  applied_margin_pct decimal(5,2) nullable

Frozen at save time by WorkOrderWork::saving hook. Recomputes only if
total OR master_id changes (i.e., someone actively edits the price or
reassigns the mechanic — in those cases we WANT the salary_base to
follow). Legacy rows (before this feature) have null salary_base;
PayrollCalculator falls back to total for them.

== PayrollCalculator uses salary_base ==

Previously: sum(wo_works.total) × works_pct → gave the mechanic a cut
of the price INCLUDING margin.

Now: sum(salary_base ?? total) × works_pct → the cut is from the
labor rate excluding margin.

Impact: for a 250 lei diagnosis at 20% margin with 50% payroll cut, the
mechanic gets 200 × 50% = 100 lei (was 250 × 50% = 125 lei). The shop
keeps the 50 lei margin regardless of the payroll %.

== RBAC gate ==

New permission FINANCE_VIEW_INTERNAL_MARGIN. Assigned to owner + admin +
manager + accountant in seed matrix. Not granted to mechanic,
receptionist, or viewer — those roles never see the "Bază salariu"
disclosure line or the margin % fields.

== UI surfaces ==

UserResource — new "Salariu & marjă" section (visible only with
FINANCE_VIEW_INTERNAL_MARGIN):
  - Tarif orar (MDL)
  - Marjă internă (%) with helper text explaining the -X% semantics
  - Placeholder tells manager the exact formula

WorkOrderResource form — new override_margin_pct field in the "Plată &
total" section, gated by same permission. Helper text: "Doar pentru
cazuri speciale. Lasă gol pentru a folosi marja mecanicului."

WorksRelationManager (WO edit page) — Total column now shows a gray
subtitle line "Bază salariu: 200.00 MDL · marjă 20%" ONLY for users
with FINANCE_VIEW_INTERNAL_MARGIN. Everyone else sees just Total.

== Contract tests: NO leak ==

InternalMarginTest verifies with black-box grepping that:
- WorkOrderPdfService::generate output contains NONE of
  {salary_base, internal_margin, applied_margin_pct, marja intern,
  Bază salariu}
- /api/track/{token} JSON payload contains NONE of the same terms
- wo_parts table has no salary_base column (margin ONLY on labor)
- Changing mechanic.internal_margin_pct after work is saved does NOT
  rewrite the historical salary_base (frozen)
- WO override wins over mechanic margin (contract-priced clients)
- Fallback chain: WO → mechanic → company default → 0

== Suite ==
298 passed (828 assertions). Was 285. +13 InternalMarginTest.

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

app/Models/Tenant/WorkOrderWork.php

@@ -45,6 +45,7 @@ class WorkOrderWork extends Model
         'mechanic_status', 'mechanic_started_at', 'mechanic_done_at',
         'actual_hours', 'paused_seconds_total', 'paused_at',
         'block_reason', 'block_note',
+        'salary_base', 'applied_margin_pct',
     ];
 
     protected $casts = [
@@ -59,6 +60,8 @@ class WorkOrderWork extends Model
         'paused_at' => 'datetime',
         'actual_hours' => 'decimal:2',
         'paused_seconds_total' => 'integer',
+        'salary_base' => 'decimal:2',
+        'applied_margin_pct' => 'decimal:2',
     ];
 
     // ── State machine ────────────────────────────────────────────
@@ -172,6 +175,16 @@ class WorkOrderWork extends Model
             if ($row->requires_approval && empty($row->approval_token)) {
                 $row->approval_token = \Illuminate\Support\Str::random(24);
             }
+            // Compute internal margin & freeze salary_base at save time.
+            // Once frozen, changing user.internal_margin_pct later does NOT rewrite history.
+            if (($row->salary_base === null || $row->isDirty(['total', 'master_id'])) && (float) $row->total > 0) {
+                $resolver = app(\App\Services\MarginResolver::class);
+                $mechanic = $row->master_id ? User::find($row->master_id) : null;
+                $wo = $row->workOrder;
+                $marginPct = $resolver->resolve($wo, $mechanic);
+                $row->applied_margin_pct = $marginPct;
+                $row->salary_base = $resolver->computeSalaryBase((float) $row->total, $marginPct);
+            }
         });
         static::saved(fn (self $row) => $row->workOrder?->recalcTotal());
         static::deleted(fn (self $row) => $row->workOrder?->recalcTotal());