feat: betting is now exact-or-nothing, not free upside (#1004)

[mholzi]

Why

Discussion #950 / issue #1004: the double-or-nothing bet had zero downside. A bet doubled the score on any points and gave 0 otherwise — but a round scoring 0 already breaks the streak and banks 0, so a "lost" bet cost nothing. Betting was free upside; the rational play was to always bet.

Change — Design A "exact or nothing" + rename to Triple or Nothing

Mechanic (scoring.py, test_scoring.py):

• A bet wins only on the exact year → round score × 3 (BET_WIN_MULTIPLIER).
• A bet on a non-exact guess → 0 for the round — forfeiting those points is the real stake.
• apply_bet_multiplier(round_score, bet, is_exact); caller passes player.years_off == 0.
• No negative scores. 52 scoring tests passing.

Copy — the feature is renamed Double → Triple or Nothing, and every user-facing string moves from ×2 / "double" / "bet you're close" to ×3 / "triple" / "exact year":

• player.html onboarding tour (title, caption, bet button, score mockup 12×3=36)
• i18n/{en,de,es,fr,nl}.json — bet, betShort, betWon, betMultiplier, betsWonDesc, bettingHint, onboarding title/caption
• README "Triple or Nothing" section + mentions

⚠️ Test note

Frontend not browser-tested — please smoke-test the onboarding tour + bet flow before merge.

Closes #1004

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request redesigns the betting mechanic from a double-or-nothing system to an "exact or nothing" gamble. Players now receive a triple-point multiplier for guessing the exact year but forfeit all points for the round if they miss the exact year while a bet is active. The changes include updates to the scoring logic, documentation in the README, and corresponding unit tests. A critical issue was identified where lost bets fail to reset the player's streak because the streak calculation incorrectly uses the pre-bet score; a fix was suggested to use the final round score instead.

[gemini-code-assist]

The new "exact or nothing" betting logic introduces a bug where a player's streak is not reset when they lose a bet on a "close" guess.

In the previous implementation, a bet only resulted in a "lost" outcome if the base score was already 0. Now, a bet can be lost even if the guess was within the scoring range (i.e., speed_score > 0).

Because _apply_streak (on line 613) is called with speed_score instead of the final player.round_score, the streak will incorrectly increment and potentially award a streak bonus despite the player scoring 0 for the round. Updating speed_score to match the post-bet round_score ensures that any round resulting in 0 points correctly resets the streak.

Suggested change

	player.round_score, player.bet_outcome = apply_bet_multiplier(
	speed_score, player.bet
	speed_score, player.bet, player.years_off == 0
	)
	player.round_score, player.bet_outcome = apply_bet_multiplier(
	speed_score, player.bet, player.years_off == 0
	)
	# Use post-bet score for streak calculation to ensure lost bets reset streaks
	speed_score = player.round_score