Add project variable migration support

[premtsd-code]

Summary

Stacks on #183. Adds project-variable as a migrable resource using the Appwrite SDK's Project::listVariables() (source) and a direct dbForProject->createDocument('variables', ...) (destination) that mirrors upstream Project::Variables::Create exactly.

Notes

• Resource::TYPE_PROJECT_VARIABLE = 'project-variable' (kebab) — placed in GROUP_INTEGRATIONS_RESOURCES alongside platform/api-key for minimum changes to other Source/Destination implementations.
• Destination writes resourceType: 'project' (singular, as upstream does for project-scope) with empty resourceInternalId/resourceId and a synthesized search field.
• Secret variables: the SDK strips value on read for secret variables, so secrets will arrive with empty values. Operator re-enters them on the destination side. (Same caveat as API key secrets.)

[greptile-apps]

Greptile Summary

This PR introduces project-variable as a migratable resource type, enabling project-level environment variables to be exported from an Appwrite source and written directly to the destination project's variables collection.

• A new ProjectVariable resource class is added under Resources/Settings/, and TYPE_PROJECT_VARIABLE is wired into Transfer::GROUP_SETTINGS_RESOURCES, ALL_PUBLIC_RESOURCES, and the extractServices match expression.
• Sources/Appwrite.php gains exportGroupSettings/exportProjectVariables (pagination mirrors exportApiKeys), and Destinations/Appwrite.php gains createProjectVariable with a pre-flight duplicate check plus DuplicateException guard.
• All non-Appwrite sources (CSV, Firebase, JSON, NHost, MockSource) implement the new abstract exportGroupSettings stub, and Source.php maps the group in its export dispatch.

Confidence Score: 5/5

Safe to merge; bulk migration works correctly end-to-end.

The implementation faithfully follows the established patterns for api-key and platform migration. The one observation — TYPE_PROJECT_VARIABLE missing from ROOT_RESOURCES — makes the per-variable rootResourceId filter unreachable but does not affect normal bulk migration.

src/Migration/Transfer.php has the ROOT_RESOURCES gap worth a follow-up if single-variable export by ID is desired.

Important Files Changed

Filename	Overview
src/Migration/Resources/Settings/ProjectVariable.php	New resource class for project-scoped variables; mirrors pattern of other resource types, correct group/name wiring.
src/Migration/Sources/Appwrite.php	Adds exportGroupSettings and exportProjectVariables; rootResourceId filter branch is unreachable because TYPE_PROJECT_VARIABLE is absent from ROOT_RESOURCES.
src/Migration/Destinations/Appwrite.php	Adds importSettingsResource and createProjectVariable; duplicate-check and DuplicateException handling look correct; follows same pattern as createApiKey/createPlatform.
src/Migration/Transfer.php	Adds GROUP_SETTINGS constant, GROUP_SETTINGS_RESOURCES, and the missing extractServices match arm; all wiring appears complete.
src/Migration/Resource.php	Adds TYPE_PROJECT_VARIABLE constant with correct kebab-case value; added to ALL_PUBLIC_RESOURCES.
src/Migration/Source.php	Adds abstract exportGroupSettings, getSettingsBatchSize helper, and GROUP_SETTINGS mapping; consistent with other group additions.
tests/Migration/Unit/Adapters/MockSource.php	Adds exportGroupSettings stub to MockSource; follows same pattern as other group stubs.

_{Reviews (6): Last reviewed commit: "Merge branch 'main' into add-project-var..." | Re-trigger Greptile}

[greptile-apps]

Both createPlatform and createApiKey call $this->dbForPlatform->purgeCachedDocument('projects', $this->project) after writing their documents. createProjectVariable omits this. If the platform's projects document caches any project-level metadata (variable counts, etc.), created variables won't be reflected until the cache naturally expires. Worth verifying against the upstream Project::Variables::Create implementation to confirm whether the purge is needed here too.

Suggested change

	} catch (DuplicateException) {
	$resource->setStatus(Resource::STATUS_SKIPPED, 'Project variable already exists');
	return false;
	}
	return true;
	}
	private function validateFieldsForIndexes(Index $resource, UtopiaDocument $table, array &$lengths)
	} catch (DuplicateException) {
	$resource->setStatus(Resource::STATUS_SKIPPED, 'Project variable already exists');
	return false;
	}
	$this->dbForPlatform->purgeCachedDocument('projects', $this->project);
	return true;
	}
	private function validateFieldsForIndexes(Index $resource, UtopiaDocument $table, array &$lengths)