Add SMTP migration

[premtsd-code]

Adds the project SMTP configuration as a new settings resource. Source reads via the typed Project::get() model; destination writes the project doc's smtp map directly (read-then-merge to preserve destination password — source API never exposes it). Stacks on top of #186.

[greptile-apps]

Greptile Summary

This PR adds project SMTP configuration as a new Settings resource type, completing the pattern established by Protocols, Labels, and Services. The source reads the typed Project model fields; the destination performs a read-then-merge on the platform projects document so the destination's stored password is preserved (the source API never exposes it).

• SMTP resource class, TYPE_SMTP constant, and entries in GROUP_SETTINGS_RESOURCES / ALL_PUBLIC_RESOURCES follow the singleton-resource pattern exactly.
• exportSMTP error handling uses the correct (int) $e->getCode() ?: Exception::CODE_INTERNAL form, consistent with all sibling exporters.
• createSMTP mirrors createProtocols (read-merge-update via dbForPlatform) with the intentional omission of the password field, documented in both the docblock and PR description.

Confidence Score: 5/5

Safe to merge — the change is a self-contained singleton resource that follows an established pattern with no regressions to existing functionality.

The implementation is structurally identical to the existing Protocols/Labels/Services singleton migrations. Error handling, status reporting, read-then-merge destination writes, and registration in Transfer/Resource constants are all consistent with the rest of the codebase. The password-omission limitation is documented and intentional.

No files require special attention.

Important Files Changed

Filename	Overview
src/Migration/Resources/Settings/SMTP.php	New singleton resource class for SMTP settings; well-typed, correctly omits password, consistent with other Settings resource classes.
src/Migration/Sources/Appwrite.php	Adds exportSMTP() and report counting; error handling uses the correct (int) cast + CODE_INTERNAL fallback, consistent with all sibling singleton exporters.
src/Migration/Destinations/Appwrite.php	createSMTP follows the same read-then-merge pattern used by createProtocols; imports TYPE_SMTP and dispatches correctly in importSettingsResource.
src/Migration/Resource.php	Adds TYPE_SMTP constant and registers it in the all-types list; placement is correct.
src/Migration/Transfer.php	Adds TYPE_SMTP to GROUP_SETTINGS_RESOURCES and ALL_PUBLIC_RESOURCES; consistent with other settings singleton types.

_{Reviews (3): Last reviewed commit: "Normalize error code for SMTP export han..." | Re-trigger Greptile}