feat: add drawer cash in/out adjustments with balance reconciliation

- New drawer_adjustment table (type: cash_in/cash_out, amount, reason)
- POST/GET /drawer/:id/adjustments endpoints
- Drawer close calculation now includes adjustments: expected = opening + sales + cash_in - cash_out
- DrawerAdjustmentSchema for input validation
- 5 new tests (44 total POS tests passing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/backend/src/db/migrations/0042_drawer-adjustments.sql

@@ -0,0 +1,14 @@
+DO $$ BEGIN
+  CREATE TYPE "adjustment_type" AS ENUM ('cash_in', 'cash_out');
+EXCEPTION WHEN duplicate_object THEN NULL;
+END $$;
+
+CREATE TABLE IF NOT EXISTS "drawer_adjustment" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+  "drawer_session_id" uuid NOT NULL REFERENCES "drawer_session"("id"),
+  "type" "adjustment_type" NOT NULL,
+  "amount" numeric(10, 2) NOT NULL,
+  "reason" text NOT NULL,
+  "created_by" uuid NOT NULL REFERENCES "user"("id"),
+  "created_at" timestamp with time zone DEFAULT now() NOT NULL
+);