docs: XML documentation pass across Domain, Service & API public surface

[Akash29g]

Summary

Adds XML documentation comments (/// <summary>, <param>, <returns>, <inheritdoc />)
across the entire public surface of the backend. This is a comments-only change —
zero logic was touched in any file. The goal is richer IntelliSense for the team and
auto-generated endpoint/model descriptions in Swagger UI.

Why

• The public API surface (controllers, service contracts, DTOs, domain entities) had little
  to no inline documentation.
• GenerateDocumentationFile is enabled for DocAnalytics.Domain and DocAnalytics.Service,
  and Swagger is wired to pull XML from all DocAnalytics.*.xml assemblies — but the doc files
  were mostly empty of summaries.
• Result: bare Swagger UI and no hover-docs in the IDE.

What changed (by layer)

Domain (DocAnalytics.Domain)

• All 17 entities documented (FileRecord, FileStepHistory, InvoiceHeader, InvoiceLineItem,
  ItemCategory, ActivityLog, AlertNotification, AlertRule, DocumentType, ErrorCatalog,
  Site, Tenant, Transaction, User, UserSiteAccess, LoginAttempt, RefreshToken).
• Common contracts: ITenantScoped, ICurrentUser — summaries explain the global
  query-filter / tenancy contract they enforce.

Service (DocAnalytics.Service)

• All feature areas covered: ActivityLog, AdminUsers, Alerts, Analytics, Auth, Aws, Batches,
  Dashboard, Errors, Extraction, Files, Health, Invoices, Provisioning, Realtime, Storage, Uploads.
• Interfaces get full summaries; implementations use /// <inheritdoc /> to avoid duplication.
• DTOs / records, DI feature-extensions, and Common helpers (PagedResult<T>,
  OneOfAttribute, DateTimeExtensions) documented.

API (DocAnalytics.Api)

• All 16 controllers.
• Realtime (PipelineHub, SignalRPipelineNotifier), Common (ApiResponse<T>,
  BaseController, CurrentUser), Auth (JwtSettings), Configuration (rate-limiting +
  security options/extensions), Extensions (ApiServiceExtensions, ValidationExtensions),
  Middleware (exception handling, security headers, tenant/site), BackgroundServices
  (ExtractionWorker, AlertEvaluationBackgroundService), and the Swagger operation filter.

Doc style

Consistent with an application (not a public library):

• One informative <summary> per public member — descriptive, not restating the name.
• <param> / <returns> where they add clarity (public methods, DI extensions, records).
• /// <inheritdoc /> on interface implementations to keep a single source of truth.

Verification

• ✅ dotnet build -warnaserror:CS1591 — build succeeds, zero warnings (proves every
  public member in the doc-file-emitting assemblies is documented).
• ✅ All existing unit/integration tests still pass (no behavioral change).
• ✅ Swagger UI now renders summaries/params for endpoints and schemas
  (AddSwaggerWithJwt → IncludeXmlComments across DocAnalytics.*.xml).

Risk

Minimal — comments only. No changes to method signatures, control flow, DI registrations,
or runtime behavior.