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/tbank/brokerage-account.dto.ts

@@ -3,29 +3,29 @@ import { MoneyDto } from '../common/money.dto';
 
 export class PortfolioSummaryDto {
   @ApiProperty({ type: MoneyDto, isArray: true })
-  totalsByCurrency: MoneyDto[];
+  declare totalsByCurrency: MoneyDto[];
 
   @ApiProperty({
     type: 'integer',
     minimum: 0,
     description: 'Number of non-zero stock positions, not the number of units.',
   })
-  stockPositionCount: number;
+  declare stockPositionCount: number;
 
   @ApiProperty({
     type: 'integer',
     minimum: 0,
     description: 'Number of non-zero bond positions, not the number of units.',
   })
-  bondPositionCount: number;
+  declare bondPositionCount: number;
 }
 
 export class BrokerageAccountDto {
   @ApiProperty()
-  id: string;
+  declare id: string;
 
   @ApiProperty()
-  name: string;
+  declare name: string;
 
   @ApiProperty({
     enum: [
@@ -35,11 +35,11 @@ export class BrokerageAccountDto {
       'MONEY_MARKET_FUND',
     ],
   })
-  type:
+  declare type:
     'BROKERAGE' | 'INDIVIDUAL_INVESTMENT' | 'INVEST_BOX' | 'MONEY_MARKET_FUND';
 
   @ApiProperty({ type: String, nullable: true, format: 'date' })
-  openedOn: string | null;
+  declare openedOn: string | null;
 
   @ApiProperty({ type: PortfolioSummaryDto, nullable: true })
   portfolioSummary?: PortfolioSummaryDto | null;