fix(wordgame): correct scoring, game-end on correct guess, and concurrent guess race

[psykzz]

Fixes #188

Summary

Three bugs in the wordgame cog:

1. Game continued after correct guess — a player who guessed the word correctly had bonus_guess_at set, so they received a bonus guess DM 5 minutes later and could keep guessing.
2. Double position points — concurrent guesses both read a stale snapshot of claimed_positions, so two players could each earn position bonus for the same (letter, position) pair.
3. Letter dedup blocked position bonus — the seen_letters continue also skipped the position-bonus check for repeated letters in the same guess (e.g. the second p in poppy would neither earn a letter bonus nor a position bonus, losing 1 pt).

Changes

• wordgame/game.py — decouple letter bonus dedup from position bonus check; both are now evaluated independently per character
• wordgame/wordgame.py — add per-thread asyncio.Lock; _process_guess now acquires the lock and re-reads fresh Config state before scoring; skip bonus_guess_at when word is guessed correctly

Testing

• Syntax validated: python -m py_compile wordgame/game.py wordgame/wordgame.py
• Scoring verified: 5-letter all-unique-correct = 12 pts (5 letter + 5 pos + 2 word), repeated-letter word like poppy = 10 pts max