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>

backend/app/models/character.py

@@ -24,6 +24,8 @@ class Character(Base):
     stats: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
     description: Mapped[str] = mapped_column(Text, nullable=False, default="")
     combat_stats: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
+    current_hp: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    max_hp: Mapped[int | None] = mapped_column(Integer, nullable=True)
     abilities: Mapped[list] = mapped_column(JSONB, nullable=False, default=list)
     equipment: Mapped[list] = mapped_column(JSONB, nullable=False, default=list)
     created_at: Mapped[datetime] = mapped_column(server_default=func.now(), nullable=False)