Per-map leaderboard filtering #92

Description

Add map-based filtering to the leaderboard page — e.g. "who's the best Factory player?"

Filter the global leaderboard by map, showing stats (kills, survival rate, raids) scoped to a specific map. Could be a dropdown/tab on the existing leaderboard page.

Context

Follow-up from #49 (raid statistics and leaderboard). The global leaderboard ships first; per-map filtering is a natural extension once data is flowing.

Dependencies

• Raid statistics and leaderboard #49 (leaderboard must exist first)