speed up slow stats aggregations

[Warbs816]

Rewrites four slow stats aggregations in apps/server/src/database/queries/stats.ts:

• getLongestListeningSession — replaces the O(N²) $reduce+$concatArrays sessionizer with a $setWindowFields+$shift pass (cumulative-sum session id).
• albumDateRatio, featRatio, getBestArtistsPer — drop the per-play tracks lookup and read albumId / artistIds / primaryArtistId straight off infos. albumDateRatio additionally pre-aggregates by (albumId, time bucket) before the albums lookup.

Output shapes and numerical results unchanged.

Measured locally on Mongo 8.3 (one user, ~85K plays, all-time year split):

Query	Before	After
getLongestListeningSession	82 s	1.6 s
albumDateRatio	7.0 s	1.8 s
featRatio	4.1 s	1.1 s
getBestArtistsPer	4.1 s	0.9 s

Requires MongoDB ≥ 5.0 (for $setWindowFields/$shift).