* Document Azure Blob Storage as a file storage backend

Adds Azure Blob Storage to the File storage system reference: a new
azureblob driver-name option and individual entries for the
FileSettings.AzureStorageAccount, AzureContainer, AzurePathPrefix,
AzureAccessKey, AzureEndpoint, AzureSSL, and
AzureRequestTimeoutMilliseconds settings. Extends the dedicated export
filestore list with the matching Export* variants.

Calls out the restart-required behaviour when changing file storage
settings so admins know Save in System Console isn't enough on its
own.

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AI assisted commit

* Add walk-through page for configuring Azure Blob Storage

Folds the full prereqs, Azure portal/CLI provisioning, System Console
walk-through, Test Connection semantics, restart-required warning,
verification, optional export backend, and troubleshooting sections
into a dedicated configure/azure-blob-storage page.

Wires the new page into the configuration-settings toctree and bullet
list, and adds a seealso link from the File storage section of
environment-configuration-settings so admins can find it from the
reference page.

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* Document migrating existing files from Amazon S3

Adds a Migrate existing files from Amazon S3 section to the Azure
Blob Storage walk-through. Covers the recommended trickle-then-cutover
pattern (long rclone sync, short AzCopy maintenance window), the
prerequisites for the migration host, phase-by-phase commands,
verification queries (object count parity, sha256 spot-check), the
rollback path, and caveats (S3 versioning, sync vs copy, prefix
rewrites, cross-region cost, Storage Mover preview status).

Updates the migration note under step 4 and the troubleshooting entry
for missing pre-cutover files to cross-reference the new section.

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* Document AzureCloud and the updated AzureEndpoint semantics

Adds the AzureCloud setting documentation (commercial/government/custom)
to both the environment configuration reference and the Azure walkthrough,
rewrites the AzureEndpoint entry to reflect its new role as the full
Blob service URL valid only when AzureCloud is custom, and removes the
stale note that said sovereign clouds aren't configured through the
endpoint override. Adds ExportAzureCloud to the dedicated export
filestore key list.

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* Document DefaultAzureCredential authentication mode

Restructures Step 3 of the Azure Blob Storage walk-through to cover
both shared-key and the new default_credential authentication mode.
The default_credential subsection walks an admin through picking the
identity source that matches the host (managed identity on Azure
VM/App Service/AKS, workload identity, service principal, az login)
and granting Storage Blob Data Contributor on the storage account.

Updates Step 4 to describe the new "Azure authentication" dropdown
in the System Console, including the conditional visibility of the
Azure Storage account key field. Adds AuthorizationPermissionMismatch
to the troubleshooting table and documents the propagation delay
that follows a fresh role assignment.

Adds FileSettings.AzureAuthMode to the environment-configuration
reference and to the ExportAzure* list under the dedicated export
filestore section.

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* Simplify the docs

* Update source/administration-guide/configure/azure-blob-storage.rst

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

* Document Azure SAS support for presigned export downloads

Add ExportAzurePresignExpiresSeconds to the dedicated export filestore
key list, and generalize the presigned-download note so it reflects that
exports can now be downloaded via an Azure Blob Storage SAS URL, not just
an Amazon S3 presigned URL.

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* Address review comments

* Add missing new line

* Fix malformed quotes

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Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>