fix(backend): Add input validation and sanitization across all DTOs

[KuchiMercy]

Description

Not all DTOs have complete class-validator decorators. Audit and harden all DTOs across all modules.

DTO locations: src/modules/escrow/dto/, admin/dto/, assets/dto/, auth/dto/
Library: class-validator and class-transformer (installed)

Acceptance Criteria

• Audit every DTO and list fields missing validation decorators
• Add @IsString(), @IsNotEmpty() to all required string fields
• Add @IsUUID() to all ID fields
• Add @MinLength() and @MaxLength() to text fields
• Add @Matches() with Stellar public key regex for address fields
• Add @IsNumber(), @Min(), @Max() to amount fields
• Add @IsOptional() to nullable fields
• Add @IsEnum() for enum-typed fields
• Enable global ValidationPipe with whitelist and forbidNonWhitelisted
• Create @IsStellarAddress() custom decorator
• Unit tests for each DTO

Points: 100

[patopatrish]

@patopatrish has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, i would love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @patopatrish to this issue.

[pharwasz]

@pharwasz has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I can work on this
Kindly assign will get it done asap

ℹ️ Repo Maintainers: To accept this application, review their application or assign @pharwasz to this issue.

[drips-wave]

Congratulations, @pharwasz! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @pharwasz: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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