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>

database/migrations/2026_07_07_000001_add_internal_margin_fields.php

@@ -0,0 +1,49 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('users', function (Blueprint $t) {
+            if (! Schema::hasColumn('users', 'internal_margin_pct')) {
+                $t->decimal('internal_margin_pct', 5, 2)->nullable()->after('hourly_rate');
+            }
+        });
+
+        Schema::table('work_orders', function (Blueprint $t) {
+            if (! Schema::hasColumn('work_orders', 'override_margin_pct')) {
+                $t->decimal('override_margin_pct', 5, 2)->nullable()->after('discount_pct');
+            }
+        });
+
+        Schema::table('wo_works', function (Blueprint $t) {
+            if (! Schema::hasColumn('wo_works', 'salary_base')) {
+                // Frozen at save time so changing margin later doesn't rewrite history
+                $t->decimal('salary_base', 10, 2)->nullable()->after('total');
+            }
+            if (! Schema::hasColumn('wo_works', 'applied_margin_pct')) {
+                // The exact margin % used when salary_base was computed
+                $t->decimal('applied_margin_pct', 5, 2)->nullable()->after('salary_base');
+            }
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('users', function (Blueprint $t) {
+            if (Schema::hasColumn('users', 'internal_margin_pct')) $t->dropColumn('internal_margin_pct');
+        });
+        Schema::table('work_orders', function (Blueprint $t) {
+            if (Schema::hasColumn('work_orders', 'override_margin_pct')) $t->dropColumn('override_margin_pct');
+        });
+        Schema::table('wo_works', function (Blueprint $t) {
+            foreach (['salary_base', 'applied_margin_pct'] as $col) {
+                if (Schema::hasColumn('wo_works', $col)) $t->dropColumn($col);
+            }
+        });
+    }
+};