fix(email): complete backfill jobs for empty mailboxes

[gbirman]

Backfill jobs are only finalized when a finished thread bumps the Redis counter to the job total, so a mailbox with zero threads stayed InProgress forever — leaving the inbox syncing and blocking resyncs (one in-flight job per link). init_backfill now completes the job directly when total_threads == 0, and list_threads does the same when the job retrieves no threads at all (covers a stale/approximate Gmail profile count). Closes 019eb3d0-de70-758e-98aa-fbb1f4584964.

[macro-application]

[email] backfill never completes for an empty mailbox

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 1108c76 and f40dbc2.

📒 Files selected for processing (3)

• rust/cloud-storage/email_service/src/pubsub/backfill/increment_counters.rs
• rust/cloud-storage/email_service/src/pubsub/backfill/init.rs
• rust/cloud-storage/email_service/src/pubsub/backfill/list_threads.rs

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