refactor: deduplicate persistence, add upsert queries, throttle snake saves

- Replace Create+Get+Update with UpsertGame/UpsertSnakeGame queries
- Extract free functions (saveGame, loadGame, etc.) from duplicated
  receiver methods on Store and Instance types
- Remove duplicate generateID from snake package, reuse game.GenerateID
- Throttle snake game DB writes to every 2s instead of every tick
- Fix double-lock in c4game chat handler
- Update all code for sqlc pointer types (*string instead of sql.NullString)

db/queries/snake_games.sql

@@ -1,16 +1,17 @@
--- name: CreateSnakeGame :one
-INSERT INTO games (id, board, current_turn, status, game_type, grid_width, grid_height, max_players, game_mode, snake_speed)
-VALUES (?, ?, 0, ?, 'snake', ?, ?, 8, ?, ?)
-RETURNING *;
+-- name: UpsertSnakeGame :exec
+INSERT INTO games (id, board, current_turn, status, game_type, grid_width, grid_height, max_players, game_mode, snake_speed, winner_user_id, rematch_game_id, score)
+VALUES (?, ?, 0, ?, 'snake', ?, ?, 8, ?, ?, ?, ?, ?)
+ON CONFLICT(id) DO UPDATE SET
+    board = excluded.board,
+    status = excluded.status,
+    winner_user_id = excluded.winner_user_id,
+    rematch_game_id = excluded.rematch_game_id,
+    score = excluded.score,
+    updated_at = CURRENT_TIMESTAMP;
 
 -- name: GetSnakeGame :one
 SELECT * FROM games WHERE id = ? AND game_type = 'snake';
 
--- name: UpdateSnakeGame :exec
-UPDATE games
-SET board = ?, status = ?, winner_user_id = ?, rematch_game_id = ?, score = ?, updated_at = CURRENT_TIMESTAMP
-WHERE id = ? AND game_type = 'snake';
-
 -- name: DeleteSnakeGame :exec
 DELETE FROM games WHERE id = ? AND game_type = 'snake';