feat: deep-link entity names in activity log and error list to batch/file detail pages

[Akash29g]

What

• Activity Log — the Entity column (e.g. Batch 335ccce0, invoice_Adam.pdf) is now a
  clickable link. Batch rows navigate to /batches/:id; File rows navigate to
  /batches/:batchId/files/:fileId.
• Error List — file names in the error table are now clickable links that navigate
  directly to the file's step-history detail page.

Why

Entity names in both pages were static text. Ops teams had to manually find the
batch/file separately — now one click takes them straight to the drill-down.

Backend changes

• ActivityLogDtos.cs — added EntityId (Guid) + BatchId (nullable Guid) to
  ActivityLogItemDto; changed class → record (required for with expression).
• ActivityLogService.cs — after fetching paged results, does a second targeted query
  on _db.Files to resolve TransactionId for File-type entries and populate BatchId.
  Guard _db.Files is not null handles MockDb test contexts.
• ErrorDtos.cs — added BatchId (nullable Guid) to ErrorListItemDto; class → record.
• ErrorService.cs — same batch-lookup pattern as activity log.

Frontend changes

• activity-log.models.ts — added entity_id + batch_id fields.
• activity-log.component.ts — injected Router + SiteContextService; added
  navigateTo() and isNavigable() methods.
• activity-log.component.html — entity column uses <button class="entity-link">
  when navigable, static text otherwise.
• activity-log.component.css — link button styles.
• Same 4-file pattern for errors/.

Testing

• All 145 service tests passing.
• Manual: Batch rows → batch detail ✅ · File rows → file step-history ✅ · Errors file names → file detail ✅

Checklist

• No schema migration needed
• Backend guard handles MockDb test context (_db.Files is not null)
• Existing tests unaffected
• Cross-review (CODEOWNERS)