feat: image variants cache panel

docs/image-variants.md

@@ -56,6 +56,31 @@ file never takes a slot. The pixel cap defaults to 50000000 pixels.
 - `generate_on_upload` is stored and returned only. Generating at upload time is not part of this
   release.
 
+### Request tracking and the Cache screen
+
+`/settings/media/cache` shows what the cache holds and purges it by scope (all, unused, one variant,
+one image). It reads `GET /v1/image-variants/cache` and writes `POST /v1/image-variants/cache/purge`
+([api.md](api.md)).
+
+Two things are tracked for it, without a database write per request. `usage.Tracker`
+(`internal/infrastructure/usage`) keeps in memory, behind the `image.VariantUsageTracker` port, the
+last time each (variant, source) file was requested and a counter of rejected requests per UTC day.
+`ServeVariantUseCase` records a hit when a cached file is served, when a file is generated by the
+request and on a 304; it counts a rejection when the request answers 400 (unknown or disabled
+variant, unsupported parameter). A missing source (404), a request without `?v=` and the admin
+generation routes record nothing. The tracker flushes one batch through `image.VariantUsageRepository`
+every 30 seconds, as soon as 500 distinct files are pending, before `GET /v1/image-variants/cache`
+reads, and on graceful shutdown; only `api/dependecyInjection.go` builds it and starts the loop. A
+failed flush keeps the pending hits and counts, so nothing is lost or counted twice. The stored
+`last_requested_at` of a file never moves back, a late update for a purged file changes nothing, and
+day rows older than 7 days are deleted by the flush.
+
+A file is unused when its reference time (`last_requested_at`, or `created_at` when never requested)
+is older than 30 days and its variant is enabled. A purge reads the rows in batches of 500 and keeps
+the rows whose blob cannot be deleted. Each scope records its own activity action
+(`media.variant_purge_all`, `media.variant_purge_unused`, `media.variant_purge`,
+`media.variant_purge_image`).
+
 ## Geometry
 
 Images are never enlarged.