feat: real step processing times + BATCH_DELETED audit trail with orphaned row dimming

[Akash29g]

What

Two features in one branch:

1 — Real Step Processing Time Percentiles

• AnalyticsService.GetStepPercentilesAsync: replaced hardcoded 120s with real
  query over FileStepHistory.StartedAt / CompletedAt
• ExtractionWorker: tracks 3 distinct timed phases (Validate, Extract, Load)
  with individual FileStepHistory entries replacing the single catch-all entry
• Fixed return type IReadOnlyList<> → List<> to match IAnalyticsService
• Fixed test: Sut(files) instead of undefined service variable

2 — BATCH_DELETED Audit Trail + UI

• UploadService.DeleteBatchAsync: writes a BATCH_DELETED ActivityLog
  entry (same DB transaction as the delete) capturing OldState, actor, timestamp
• ActivityLogComponent: BATCH_DELETED rows show strikethrough + 🗑️ badge
• DataTableComponent: new optional rowClass input for per-row CSS classes
• File rows whose parent batch was deleted (batch_id == null) are automatically
  dimmed (opacity: 0.4) — no extra API call needed

Testing

• Deleted a batch → activity log shows BATCH_DELETED row + all file rows grayed
• Uploaded files → Processing Time table shows real Validate / Extract / Load times
• All CI checks passing