Add Phase 1 MVP: D&D text-adventure server

FastAPI + Postgres backend with fastapi-users auth, games/characters
CRUD, and a WebSocket chat endpoint where an x.ai Grok DM narrates
play, rolls dice, and maintains character sheets via tool calls.
React + Tailwind frontend covers registration/login, game list and
creation, participation-code join flow, character viewer, and a
real-time chat session view. Docker Compose skeleton (Postgres +
backend) included; Caddy/frontend container wiring is deferred to
the deploy milestone.

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

backend/app/llm/context.py

@@ -0,0 +1,59 @@
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.character import Character
+from app.models.message import Message
+from app.models.user import User
+
+# Rough char-based budget for Phase 1 (~4 chars/token, leaves headroom for system prompt + output).
+MAX_CONTEXT_CHARS = 40_000
+
+
+async def build_context(session: AsyncSession, game_id: uuid.UUID) -> list[dict]:
+    messages = (
+        await session.execute(
+            select(Message).where(Message.game_id == game_id).order_by(Message.id.asc())
+        )
+    ).scalars().all()
+    if not messages:
+        return []
+
+    user_ids = {m.user_id for m in messages if m.user_id is not None}
+    character_ids = {m.character_id for m in messages if m.character_id is not None}
+
+    names: dict[uuid.UUID, str] = {}
+    if user_ids:
+        rows = (await session.execute(select(User.id, User.name).where(User.id.in_(user_ids)))).all()
+        names = {row.id: row.name for row in rows}
+
+    char_names: dict[uuid.UUID, str] = {}
+    if character_ids:
+        rows = (
+            await session.execute(select(Character.id, Character.name).where(Character.id.in_(character_ids)))
+        ).all()
+        char_names = {row.id: row.name for row in rows}
+
+    entries: list[dict] = []
+    for m in messages:
+        if m.sender_type == "dm":
+            entries.append({"role": "assistant", "content": m.content})
+        elif m.sender_type == "player":
+            player_name = names.get(m.user_id, "Spieler") if m.user_id else "Spieler"
+            character_name = char_names.get(m.character_id) if m.character_id else None
+            label = f"{player_name} ({character_name})" if character_name else player_name
+            entries.append({"role": "user", "content": f"{label}: {m.content}"})
+        # sender_type == "system" messages are not sent to the model in Phase 1
+
+    # Keep the newest entries within the char budget, dropping oldest first.
+    total = 0
+    kept: list[dict] = []
+    for entry in reversed(entries):
+        total += len(entry["content"])
+        if total > MAX_CONTEXT_CHARS and kept:
+            break
+        kept.append(entry)
+    kept.reverse()
+
+    return kept