Backfill Av1

Enqueue AV1 delivery transcodes for this org's existing library.

Requires org.av1_delivery_enabled (403 otherwise — prevents accidental transcode spend for non-cohort orgs). Eligible: published+ready video with a Mux asset, not protected, no av1_mappings row yet, and within settings["av1_content_tags"] scope if configured. Idempotent — named Cloud Tasks and the mapping-row PK both dedupe; paginate with limit+offset until enqueued == 0. Throughput is bounded by the av1-transcode-jobs queue (max 5 concurrent dispatches).