perf: extend action c_u index to include created_unix for faster dashboard feeds

[bircni]

Adds created_unix as the third column of the c_u composite index on the action table, changing it from (user_id, is_deleted) to (user_id, is_deleted, created_unix).

Migration 337 drops and recreates the index. No data is touched.

Root causes

#32333 introduced the c_u index to speed up dashboard queries, but defined it as (user_id, is_deleted) — without created_unix.

#3368 The simple query is now efficient enough for the database to actually use c_u, but because created_unix is absent from the index, the database must load and sort every matching row before returning the first page of 20.

The existing c_u_d index (created_unix, user_id, is_deleted) does not help because its leading column is created_unix, which can't be used for an equality seek on user_id.

Those two caused this issue: #38075

With the fix, the database seeks directly to (user_id=X, is_deleted=false) and walks created_unix in descending order, stopping after 20 rows.

Fixes #38075

[lunny]

What's the different between this index and c_u_d?

[bircni]

What's the different between this index and c_u_d?

c_u_d leads with created_unix, so it can't seek the dashboard query (which filters user_id, is_deleted and has no predicate on time). The new c_u leads with user_id, is_deleted and trails created_unix, so the DB seeks the filter and walks the trailing column in order — satisfying ORDER BY created_unix DESC LIMIT 20 without a sort. Same columns, opposite access patterns; not redundant.

[lunny]

Please resolve the conflicts.

[bircni]

Please resolve the conflicts.

@lunny fixed

[bircni]

@lunny ping