Add monster HP tracking, separate from the game-ending logic

Only the player character's HP was tracked before, so a monster's
death was never detected — only the DM's own (unenforced) narration
of it. Add a new update_monster_hp tool the DM calls whenever an
NPC/monster is introduced or takes damage, stored as DM-only
bookkeeping on Game (never exposed via GameRead, since HP/AC of NPCs
is meant to stay secret from players). The tool reports back
"defeated": true once HP drops to 0 so the DM can react to it — but
unlike a character dying, this does NOT auto-end the session; whether
a monster's death should end the game is a separate decision left to
the DM's own end_game call, or to a future rule.

Building an isolated test for this caught a real bug along the way:
the update mutated the matched monster's dict in place before
reassigning the list, which made SQLAlchemy's old-vs-new JSONB
comparison see identical content and silently skip writing the
change. Fixed by building fresh dicts instead of mutating shared ones.

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

backend/app/llm/orchestrator.py

@@ -9,7 +9,7 @@ from app.config import settings
 from app.llm.client import get_dm_system_prompt, get_llm_client
 from app.llm.context import build_context
 from app.llm.json_utils import fix_double_escaped_unicode
-from app.llm.tools import character_sheet, dice, end_game
+from app.llm.tools import character_sheet, dice, end_game, monster
 from app.models.game import Game
 from app.models.message import Message
 from app.rag.retrieval import build_rag_block
@@ -23,6 +23,7 @@ TOOLS = [
     {"type": "function", "function": dice.TOOL_SCHEMA},
     {"type": "function", "function": character_sheet.TOOL_SCHEMA},
     {"type": "function", "function": end_game.TOOL_SCHEMA},
+    {"type": "function", "function": monster.TOOL_SCHEMA},
 ]
 
 
@@ -38,6 +39,8 @@ async def _execute_tool_call(
             return await character_sheet.upsert(session, game_id, tool_input)
         if tool_name == "end_game":
             return await end_game.end(session, game_id, tool_input)
+        if tool_name == "update_monster_hp":
+            return await monster.update(session, game_id, tool_input)
         return {"error": f"Unknown tool {tool_name!r}"}
     except Exception as exc:  # noqa: BLE001
         logger.warning("Tool call %s failed: %s", tool_name, exc)