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/repository/users.sql.go

@@ -16,12 +16,12 @@ RETURNING id, username, password_hash, created_at
 `
 
 type CreateUserParams struct {
-	ID           string
-	Username     string
-	PasswordHash string
+	ID           string `db:"id" json:"id"`
+	Username     string `db:"username" json:"username"`
+	PasswordHash string `db:"password_hash" json:"password_hash"`
 }
 
-func (q *Queries) CreateUser(ctx context.Context, arg CreateUserParams) (User, error) {
+func (q *Queries) CreateUser(ctx context.Context, arg CreateUserParams) (*User, error) {
 	row := q.db.QueryRowContext(ctx, createUser, arg.ID, arg.Username, arg.PasswordHash)
 	var i User
 	err := row.Scan(
@@ -30,14 +30,14 @@ func (q *Queries) CreateUser(ctx context.Context, arg CreateUserParams) (User, e
 		&i.PasswordHash,
 		&i.CreatedAt,
 	)
-	return i, err
+	return &i, err
 }
 
 const getUserByID = `-- name: GetUserByID :one
 SELECT id, username, password_hash, created_at FROM users WHERE id = ?
 `
 
-func (q *Queries) GetUserByID(ctx context.Context, id string) (User, error) {
+func (q *Queries) GetUserByID(ctx context.Context, id string) (*User, error) {
 	row := q.db.QueryRowContext(ctx, getUserByID, id)
 	var i User
 	err := row.Scan(
@@ -46,14 +46,14 @@ func (q *Queries) GetUserByID(ctx context.Context, id string) (User, error) {
 		&i.PasswordHash,
 		&i.CreatedAt,
 	)
-	return i, err
+	return &i, err
 }
 
 const getUserByUsername = `-- name: GetUserByUsername :one
 SELECT id, username, password_hash, created_at FROM users WHERE username = ?
 `
 
-func (q *Queries) GetUserByUsername(ctx context.Context, username string) (User, error) {
+func (q *Queries) GetUserByUsername(ctx context.Context, username string) (*User, error) {
 	row := q.db.QueryRowContext(ctx, getUserByUsername, username)
 	var i User
 	err := row.Scan(
@@ -62,5 +62,5 @@ func (q *Queries) GetUserByUsername(ctx context.Context, username string) (User,
 		&i.PasswordHash,
 		&i.CreatedAt,
 	)
-	return i, err
+	return &i, err
 }