chore(migrations): renumber 021-023 to 023-025 to clear the collision with main

main independently shipped 021_hashtag_counts_respect_bans and
022_client_upload_idempotency. sqlx::migrate! embeds ./migrations and refuses two
files per version, so these three had to move before the branch could merge at all.

Contents are untouched — only the version prefixes change. Renumbering (rather than
renumbering main's) is the safe direction: main is already deployed, so its 021 and
022 are applied in production and their versions are now immutable.

backend/migrations/024_feed_scalar_counts.down.sql

@@ -0,0 +1,26 @@
+-- Restore the 016 definition verbatim.
+DROP VIEW IF EXISTS v_feed;
+CREATE VIEW v_feed AS
+SELECT
+    u.id,
+    u.event_id,
+    u.user_id,
+    usr.display_name             AS uploader_name,
+    usr.is_banned,
+    usr.uploads_hidden,
+    u.preview_path,
+    u.thumbnail_path,
+    u.display_path,
+    u.mime_type,
+    u.caption,
+    u.created_at,
+    COUNT(DISTINCT l.user_id)    AS like_count,
+    COUNT(DISTINCT c.id)         AS comment_count
+FROM upload u
+JOIN "user" usr ON u.user_id = usr.id
+LEFT JOIN "like" l  ON l.upload_id = u.id
+LEFT JOIN comment c ON c.upload_id = u.id AND c.deleted_at IS NULL
+WHERE u.deleted_at IS NULL
+  AND usr.uploads_hidden = FALSE
+  AND usr.is_banned = FALSE
+GROUP BY u.id, usr.display_name, usr.is_banned, usr.uploads_hidden;