refactor: enforce strict backend typing

Enable strict TypeScript and reject non-null and definite-assignment assertions. Declare externally populated DTO fields, preserve runtime validation, and narrow values at their owners.

Verified: backend verify (257 tests), unchanged OpenAPI, and 21 SQLite/HTTP tests on retry. The initial SQLite run had two timeouts; baseline HEAD passed 21 tests as well.

src/common/decorators/api-safe-error-response.decorator.ts

@@ -8,10 +8,10 @@ import {
 
 export class ProblemDetailsDto {
   @ApiProperty({ example: 401 })
-  status: number;
+  declare status: number;
 
   @ApiProperty({ example: 'UNAUTHORIZED' })
-  code: string;
+  declare code: string;
 
   @ApiProperty({
     required: false,

src/common/dto-validation.spec.ts

@@ -0,0 +1,61 @@
+import { BadRequestException, ValidationPipe } from '@nestjs/common';
+import { CredentialsDto } from '../auth/auth.dto';
+import {
+  CreateBankCardDto,
+  UpdateBankCardDto,
+} from '../bank-cards/bank-card.dto';
+
+describe('DTO validation', () => {
+  const pipe = new ValidationPipe({ transform: true, whitelist: true });
+
+  it('requires credentials and normalizes a valid email', async () => {
+    const metadata = { type: 'body' as const, metatype: CredentialsDto };
+    await expect(pipe.transform({}, metadata)).rejects.toBeInstanceOf(
+      BadRequestException,
+    );
+    await expect(
+      pipe.transform({ email: 'user@example.com' }, metadata),
+    ).rejects.toBeInstanceOf(BadRequestException);
+    await expect(
+      pipe.transform(
+        { email: ' User@Example.com ', password: 'long-enough-password' },
+        metadata,
+      ),
+    ).resolves.toEqual({
+      email: 'user@example.com',
+      password: 'long-enough-password',
+    });
+  });
+
+  it('requires and validates nested money in a new card', async () => {
+    const metadata = { type: 'body' as const, metatype: CreateBankCardDto };
+    const card = {
+      id: '00000000-0000-4000-8000-000000000001',
+      name: 'Everyday',
+      openingDate: '2026-09-13',
+    };
+    for (const openingBalance of [undefined, null, {}, { currency: 'RUB' }]) {
+      await expect(
+        pipe.transform({ ...card, openingBalance }, metadata),
+      ).rejects.toBeInstanceOf(BadRequestException);
+    }
+    const valid = {
+      ...card,
+      openingBalance: { currency: 'RUB', value: '100.00' },
+    };
+    await expect(pipe.transform(valid, metadata)).resolves.toEqual(valid);
+  });
+
+  it('keeps update fields optional while requiring version and rejecting null', async () => {
+    const metadata = { type: 'body' as const, metatype: UpdateBankCardDto };
+    await expect(pipe.transform({}, metadata)).rejects.toBeInstanceOf(
+      BadRequestException,
+    );
+    await expect(
+      pipe.transform({ version: 1, name: null }, metadata),
+    ).rejects.toBeInstanceOf(BadRequestException);
+    await expect(
+      pipe.transform({ version: 1 }, metadata),
+    ).resolves.toMatchObject({ version: 1 });
+  });
+});

src/common/money.dto.ts

@@ -2,8 +2,8 @@ import { ApiProperty } from '@nestjs/swagger';
 
 export class MoneyDto {
   @ApiProperty()
-  currency: string;
+  declare currency: string;
 
   @ApiProperty()
-  value: string;
+  declare value: string;
 }