feat(inbound): AttachmentDownloader for provider-hosted attachments

[mpge]

Summary

Ports the Go (escalated-go#35) and .NET (escalated-dotnet#29) reference to Spring Boot. Host apps now have a ready-to-wire worker for the Mailgun-style provider-hosted attachments surfaced in `InboundEmailService.ProcessResult.pendingAttachmentDownloads`.

Design

`AttachmentDownloader`

• `download(pending, ticketId, replyId?)` — HTTP GET via JDK `HttpClient` + persist via `AttachmentStorage` + new row on `AttachmentRepository`.
• `downloadAll` continues past per-attachment failures so a bad URL doesn't block the rest; returns a `Result` per input (`persisted` or `error` set).
• `Options.maxBytes` size cap (throws `AttachmentTooLargeException`); `Options.basicAuth` for providers that gate URLs by API key (Mailgun).
• `safeFilename` uses `Path.getFileName` so `../../etc/passwd` becomes `passwd` — crafted names can't escape the storage root.
• Response Content-Type fallback when the `PendingAttachment`'s own `contentType` is empty.

`AttachmentStorage` contract + reference `LocalFileAttachmentStorage` that writes to local FS with timestamp+nanos prefixing so concurrent uploads with the same original name don't collide. Host apps with S3 / GCS / Azure can implement `AttachmentStorage` themselves.

Tests

13 Mockito / AssertJ cases:

• `download`: happy path (ticket + reply attachable targets), 404 throw + no-persist, oversize → `AttachmentTooLargeException`, basic auth header encoding, missing URL guard, response Content-Type fallback.
• `safeFilename`: `../../etc/passwd`, absolute paths, empty, null, `..`, `.` — all neutralized.
• `downloadAll`: 3-attachment batch where the middle one fails — other two still persist.
• `LocalFileAttachmentStorage`: write, null-root rejection, unique path per call.

Uses a `StubResponse` implementing `HttpResponse<byte[]>` directly to avoid the ceremony of standing up MockWebServer for a unit test.

Stacked PR

Based on `feat/inbound-email-orchestration` (#29) so `PendingAttachment` + `ProcessResult` are available. Merge order: #26 → #27 → #28 → #29 → this PR.