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## Problem We lack an API for warming up attached locations based on the heatmap contents. This is problematic in two places: 1. If we manually migrate and cut over while the secondary is still cold 2. When we re-attach a previously offloaded tenant ## Summary of changes https://github.com/neondatabase/neon/pull/10597 made heatmap generation additive across migrations, so we won't clobber it a after a cold migration. This allows us to implement: 1. An endpoint for downloading all missing heatmap layers on the pageserver: `/v1/tenant/:tenant_shard_id/timeline/:timeline_id/download_heatmap_layers`. Only one such operation per timeline is allowed at any given time. The granularity is tenant shard. 2. An endpoint to the storage controller to trigger the downloads on the pageserver: `/v1/tenant/:tenant_shard_id/timeline/:timeline_id/download_heatmap_layers`. This works both at tenant and tenant shard level. If an unsharded tenant id is provided, the operation is started on all shards, otherwise only the specified shard. 3. A storcon cli command. Again, tenant and tenant-shard level granularities are supported. Cplane will call into storcon and trigger the downloads for all shards. When we want to rescue a migration, we will use storcon cli targeting the specific tenant shard. Related: https://github.com/neondatabase/neon/issues/10541