Plugin Directory: Strengthen duplicate-submission prevention on upload

[obenland]

Follow-up to the duplicate-submission hardening in #698.

The per-user upload lock keeps a single submitter from creating duplicates via double-clicks or reloads, but the final duplicate check and post creation aren't otherwise coordinated, so two submissions of the same new plugin can each pass the earlier checks and land as separate queue entries.

This adds a short-lived, per-slug guard (wp_cache_add, same primitive as the existing lock) around the final check-and-create step: the slug is claimed for the moment it takes to re-check for an existing plugin and create the post, then released. If the slug is already claimed, or a matching plugin now exists, the submission is rejected with a transient "currently being processed, please try again in a moment" notice — on retry, the regular duplicate checks then produce the appropriate permanent error.

Scope:

• New submissions only; updates to existing plugins are untouched.
• No change to the checks themselves, just their coordination.
• Reuses the existing object-cache primitive, so no new infrastructure.

Result: fewer duplicate entries in the review queue and less manual triage for the plugins team.

🤖 Generated with Claude Code

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