Add shared Swagger DTOs, audit diff helper, activity feed mapper, evidence validation

[benzeneCLI]

A lot of the ground for these four issues already exists (Swagger is already configured in main.ts, AdminAuditLogService already has create/findAll with filters+pagination, and analytics.service.ts already has summary/volume/users/disputes/top-users with in-memory TTL caching). This PR adds the narrower pieces that were still missing, each as its own new file:

• config::swagger-responses.dto: shared ErrorResponseDto/PaginationMetaDto so controllers can reference one consistent @ApiResponse shape instead of each hand-rolling it. Decorating every controller/handler per the full acceptance criteria is a much larger follow-up.
• admin::audit-diff.helper: buildStateDiff computing a previousState/newState diff to embed in the existing audit log metadata field, plus the missing piece toward a per-escrow history endpoint.
• admin::analytics-activity.helper: toRecentActivityFeed mapping AdminAuditLog rows to the shape a GET /admin/analytics/activity route needs — the one endpoint from the issue not already present alongside summary/volume/users/disputes/top-users.
• escrow::evidence-file-validation: size + MIME-type validators for evidence uploads, which EvidenceFileMetadataDto currently doesn't enforce.

Closes #405
Closes #413
Closes #488
Closes #489

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