feat: add show-linking-usage support CLI command

[Sneagan]

Summary

Adds bat-go skus show-linking-usage, a read-only support CLI command that shows
how many device linking slots are in use for a premium order.

Support asked for a way to see a subscriber's slot usage without touching anything.
The deletion flow (reset-linking-limit) already reveals this during its
confirmation step; this command exposes the same listing — active credential
batches via the signed GET /v1/orders/{orderID}/credentials/batches endpoint,
where each batch is one linked device — without any mutation or confirmation
prompt. The order is identified by --order-id or --email (subscriptions
support API lookup, with interactive selection when several match), optionally
scoped with --item-id.

The shared logic between the two commands is extracted into helpers used by both
(requireOrderRef, resolveOrderID, batchesURL, formatBatchTable) rather than
duplicated; reset-linking-limit's behavior and output are unchanged.

Note: the report shows slots in use only, not used-of-total — the batches
endpoint does not return the item's max_active_batches_tlv2_creds, so a
used/total display would need a small SKUs server change. Possible follow-up.

The new command's flags are deliberately not bound to viper: reset-linking-limit
already binds the shared global keys, and viper.BindPFlag is global, so rebinding
would clobber that command. Flags are read directly from cobra with env fallback.

Type of Change

• Product feature
• Bug fix
• Performance improvement
• Refactor
• Other

Tested Environments

• Development
• Staging
• Production

Before Requesting Review

• Does your code build cleanly without any errors or warnings?
• Have you used auto closing keywords?
• Have you added tests for new functionality?
• Have validated query efficiency for new database queries? (n/a — no DB queries; reuses the existing signed SKUs
  endpoint)
• Have documented new functionality in README or in comments?
• Have you squashed all intermediate commits?
• Is there a clear title that explains what the PR does?
• Have you used intuitive function, variable and other naming?
• Have you requested security and/or privacy review if needed
• Have you performed a self review of this PR?

Manual Test Plan

bat-go skus show-linking-usage
--skus-base-url https://
--private-key
--email subscriber@example.com
--subscriptions-base-url https://
--subscriptions-token <SUPPORT_TOKEN>

• Resolves the order by email (prompts when several match) and prints the slot
  count plus a per-device table (request_id, oldest_valid_from), making no changes.
• --order-id <uuid> targets an order directly without the email lookup.
• --item-id <uuid> scopes the listing to one order item.
• reset-linking-limit is unaffected: same prompts, output, and behavior as before.