refactor(loyalty): remove what is left of tokens

Tokens are EnakCoin and no app uses the token names any more, so their
compatibility layer goes:

- GET /customer/tokens and its handler, service, processor and response
  types.
- total_tokens and tokens_history on GET /customer/wallet; last_updated
  now comes from the most recent row of either currency.
- token_used and tokens_remaining on game and spin responses, and
  sort_by=token_used on the game play list.
- TOKENS as a campaign type and reward type, with the mapping to COINS:
  migration 000092 already renamed the stored values.

The customer_tokens table and its entity stay, as cmd/wallet-migrate still
reads them, and LEGACY_TOKENS stays as the reference of the MIGRATION rows
it wrote. The docs list the removed names and their replacements.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/contract/spin_game_contract.go

@@ -17,6 +17,4 @@ type SpinGameResponseData struct {
 	GamePlay       GamePlayResponse           `json:"game_play"`
 	PrizeWon       *CustomerGamePrizeResponse `json:"prize_won,omitempty"`
 	CoinsRemaining int64                      `json:"coins_remaining"`
-	// Deprecated: the same as CoinsRemaining, kept until the apps read coins_remaining.
-	TokensRemaining int64 `json:"tokens_remaining"`
 }