feat(inbound): AttachmentDownloader for provider-hosted attachments

[mpge]

Summary

Ports the Go reference (escalated-go#35) to .NET. Host apps now have a ready-to-wire worker for the Mailgun-style provider-hosted attachments surfaced in `ProcessResult.PendingAttachmentDownloads` — before this PR those URLs were returned but never fetched.

Design

`AttachmentDownloader`

• `DownloadAsync(pending, ticketId, replyId?)` → HTTP GET + persist via `IAttachmentStorage` + `Attachment` row on the EF Core context.
• `DownloadAllAsync` continues past per-attachment failures so a bad URL doesn't block the rest; returns an `AttachmentDownloadResult` per input with either `Persisted` or `Error` set.
• `AttachmentDownloaderOptions` controls `MaxBytes` size cap (rejects with the `AttachmentTooLargeException` sentinel) and optional `BasicAuth` credentials for providers that gate URLs by API key (Mailgun).
• `SafeFilename` uses `Path.GetFileName` so `../../etc/passwd` becomes `passwd` — crafted attachment names can't escape the storage root.
• Response `Content-Type` fallback when the `PendingAttachment`'s own `ContentType` is empty.

`IAttachmentStorage`

• Minimal contract (`Name` + `PutAsync`). Host apps with S3/Azure/GCS wire their own adapter; the reference `LocalFileAttachmentStorage` writes to local disk with timestamp+ticks filename prefixing so concurrent uploads with the same original name don't collide.

Tests

12 xUnit cases:

• `DownloadAsync`: happy path (ticket + reply attachable targets), 404 → throw, oversize → `AttachmentTooLargeException`, basic auth header encoding, missing URL guard, response Content-Type fallback.
• `SafeFilename`: `[Theory]` with `../../etc/passwd`, absolute paths, empty, null, `..` → all neutralized.
• `DownloadAllAsync`: 3-attachment batch where the middle one fails — other two still persist.
• `LocalFileAttachmentStorage`: file write, empty-root rejection, unique path per call.

Stacked PR

Based on `feat/inbound-email-orchestration` (#26) so `PendingAttachment` + `ProcessResult` are available. Merge order: #23 → #24 → #25 → #26 → this PR.