Leaderboards (#43)

* leaderboards

* [leaderboard] add basic leaderboards - will probably have to clean up later but data is collected now

apps/leaderboard/migrations/0001_leaderboard.sql

@@ -0,0 +1,29 @@
+-- A room's leaderboards: each player's value on each of a room's stat channels. Owned by
+-- the `leaderboard` worker, which is the only reader and the only writer —
+-- `POST /leaderboard/CheckAndSetStat` writes a row and the three reads
+-- (`GetRanks`, `GetNearbyScores`, `GetPlayerRank`) rank them. Generated from
+-- src/leaderboard-db.ts (SCHEMA_DDL) — keep in sync.
+--
+-- One row per (player, room, channel). A row is created the first time a player posts a
+-- stat for that channel of the room; a player with no row is simply not on that board,
+-- which the reads answer as an empty slice / the unranked sentinel rather than by
+-- inserting on a read.
+--
+-- `stat_channel` is the client's `StatChannel` — which of the room's tracked stats this
+-- is (a room keeps one board per channel) — and `stat_value` is what it posts as
+-- `StatValue`. What a channel counts (wins, laps, a time) is the room's business; the
+-- server only orders on it, highest first unless the client asks for ascending, ties
+-- broken on the lower `player_id` so a rank is stable between two reads.
+
+CREATE TABLE IF NOT EXISTS leaderboard (
+  player_id INTEGER NOT NULL,
+  room_id INTEGER NOT NULL,
+  stat_channel INTEGER NOT NULL,
+  stat_value INTEGER NOT NULL DEFAULT 0,
+  PRIMARY KEY (player_id, room_id, stat_channel)
+  );
+
+-- The reads walk one board (room + channel) ordered by value; the primary key serves the
+-- point lookup but not that scan.
+CREATE INDEX IF NOT EXISTS leaderboard_board_value
+  ON leaderboard (room_id, stat_channel, stat_value DESC, player_id);