Let the DM reuse an existing character instead of re-creating it

Characters are already owned by the player, not the game, and were
meant to be reusable — but the DM had no way to actually retrieve one.
upsert_character_sheet is write-only, so telling the DM "I'll bring my
character Tillo Eichenherz" led it to create a second, blank
"Tillo Eichenherz" (empty stats, thinner ability text) instead of
reusing the real one, then ask for all six ability scores again as if
nothing existed.

Two fixes:

1. New link_existing_character tool: given a player name and a
   character name, looks it up among that player's own characters
   (ownership enforced — never someone else's) and links it to
   game_participants.character_id, returning the full sheet so the DM
   can continue with real data immediately. System prompt now tells
   the DM to call this the moment a player names a previously-played
   character, instead of asking for stats.

2. run_dm_turn now auto-injects the full sheet of every already-linked
   character into the system prompt every turn (name, stats, combat
   stats, HP, abilities, equipment, background), with an explicit
   "already exists, don't ask again" instruction — this is the actual
   root-cause fix: even a character linked some other way (the
   existing join-with-character_id flow, or a future UI) now stays
   visible to the DM turn after turn instead of only living in
   whatever's left of the truncated chat window.

Also manually repaired the user's real "Schwarze Segel" game, which
had already hit this bug — re-pointed its participant back to the
real Tillo Eichenherz instead of the accidental duplicate (left the
duplicate in place rather than deleting data; it's a normal delete
away via the character-delete feature if unwanted).

Verified live against x.ai: a fresh game where the player said "Ich
nehme meinen alten Charakter Tillo Eichenherz" linked to the real
character on the first message (no duplicate created — confirmed by
row count), and a follow-up question about AC/Strength got the exact
real values (RK 18, Stärke 15) pulled from the injected sheet, not
re-asked or invented.

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

backend/app/llm/orchestrator.py

@@ -10,7 +10,8 @@ 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, monster, world_state
+from app.llm.tools import character_sheet, dice, end_game, link_existing_character, monster, world_state
+from app.models.character import Character
 from app.models.game import Game, GameParticipant
 from app.models.message import Message
 from app.models.user import User
@@ -28,6 +29,7 @@ TOOLS = [
     {"type": "function", "function": end_game.TOOL_SCHEMA},
     {"type": "function", "function": monster.TOOL_SCHEMA},
     {"type": "function", "function": world_state.TOOL_SCHEMA},
+    {"type": "function", "function": link_existing_character.TOOL_SCHEMA},
 ]
 
 
@@ -47,6 +49,8 @@ async def _execute_tool_call(
             return await monster.update(session, game_id, tool_input)
         if tool_name == "update_world_state":
             return await world_state.update(session, game_id, tool_input)
+        if tool_name == "link_existing_character":
+            return await link_existing_character.link(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)
@@ -82,6 +86,29 @@ async def _request_player_roll(
     }
 
 
+def _format_character_sheet(character: Character) -> str:
+    parts = [
+        f"- {character.name} ({character.race or '?'} {character.char_class or '?'}, Stufe {character.level})"
+    ]
+    if character.stats:
+        parts.append(f"  Werte: {character.stats}")
+    hp = None
+    if character.current_hp is not None or character.max_hp is not None:
+        hp = f"{character.current_hp if character.current_hp is not None else '?'}/{character.max_hp if character.max_hp is not None else '?'}"
+    if hp:
+        parts.append(f"  TP: {hp}")
+    if character.combat_stats:
+        parts.append(f"  Kampfwerte: {character.combat_stats}")
+    if character.abilities:
+        ability_names = ", ".join(a.get("name", "") for a in character.abilities if isinstance(a, dict))
+        parts.append(f"  Fähigkeiten: {ability_names}")
+    if character.equipment:
+        parts.append(f"  Ausrüstung: {', '.join(character.equipment)}")
+    if character.description:
+        parts.append(f"  Hintergrund: {character.description}")
+    return "\n".join(parts)
+
+
 async def _check_hp_game_over(session: AsyncSession, game_id: uuid.UUID, result: dict) -> str | None:
     """After an upsert_character_sheet call, auto-ends the game if the character it touched
     dropped to 0 HP or below. Returns the end reason if the game was just ended, else None."""
@@ -159,6 +186,21 @@ async def run_dm_turn(
                 "über den Einladungslink dazukommen; du erkennst sie automatisch am Namen, sobald sie schreiben."
             )
 
+        linked_characters = (
+            await session.execute(
+                select(Character)
+                .join(GameParticipant, GameParticipant.character_id == Character.id)
+                .where(GameParticipant.game_id == game_id)
+            )
+        ).scalars().all()
+        if linked_characters:
+            sheets = "\n".join(_format_character_sheet(c) for c in linked_characters)
+            system_prompt += (
+                "\n\nBekannte Charakterblätter (bereits vollständig vorhanden — NICHT erneut nach "
+                "Werten/Ausrüstung/Hintergrund fragen, nur bei tatsächlichen Änderungen über "
+                f"upsert_character_sheet aktualisieren):\n{sheets}"
+            )
+
     await world_state.refresh_if_due(session, game_id, client, settings.dm_model)
     world_summary = await world_state.get_summary(session, game_id)
     if world_summary: