Add game end status with an end_game DM tool and automatic 0-HP detection

Games now have a status/ended_reason pair. The DM can call the new
end_game tool when the story reaches a real conclusion (victory,
defeat, or a resolved one-shot), and the backend independently ends
the game whenever a character's HP drops to 0 or below, regardless of
whether the DM narrates it. The frontend shows a banner and a
"Beendet" badge once a game ends.

HP moves from the freeform combat_stats bag into dedicated
current_hp/max_hp columns on Character, since reliably detecting 0 HP
requires a real integer rather than parsing strings like "3/10" out of
an LLM-authored key/value dict. Also fixed combat_stats to fully
replace on each upsert instead of merging, matching its documented
contract — the merge was leaving stale keys (old HP/TP text) behind
after the model stopped sending them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/GameCard.tsx

@@ -12,6 +12,11 @@ export default function GameCard({ game }: { game: Game }) {
     >
       <div className="flex items-start justify-between gap-4">
         <h2 className="text-lg font-semibold text-slate-100">{game.name}</h2>
+        {game.status === "ended" && (
+          <span className="shrink-0 rounded bg-slate-700 px-2 py-1 text-xs font-semibold text-slate-300">
+            Beendet
+          </span>
+        )}
         {game.participation_code && (
           <span className="shrink-0 rounded bg-amber-500/20 px-2 py-1 font-mono text-xs text-amber-300">
             Code: {game.participation_code}