Add per-game adventure text with RAG-based retrieval

Players can now paste the full text of a freely available adventure
when creating a game, and the DM will follow it instead of inventing
its own plot/NPCs/locations. Reuses the existing rulebook RAG
pipeline (chunking, local embedding) rather than injecting the raw
text into every turn, since real adventure modules range from a few
pages to hundreds — far beyond what fits in a prompt.

- New adventure_chunks table (game-scoped, unlike the global
  rulebook_chunks) + Game.adventure_text storing the raw source.
- ingest_adventure_text() chunks/embeds synchronously during
  POST /api/games when adventure_text is non-empty.
- build_adventure_rag_block() retrieves by similarity to the latest
  player message, scoped to game_id — plus always includes chunk 0
  (the adventure's opening) regardless of the query, since early
  Session-0 messages rarely resemble the adventure's actual hook and
  pure similarity search could miss the beginning entirely.
- System prompt instructs the DM to follow provided adventure
  excerpts strictly, deviating only when players clearly go off-script.
- Frontend: optional adventure-text field in the game creation form,
  a has_adventure flag surfaced as a small badge on the game card and
  session header.

Verified with an isolated two-game test that chunks/retrieval never
leak across games (the critical failure mode here), and live against
x.ai: a custom-written one-shot's specific NPCs, location, and hook
appeared verbatim in the DM's opening scene instead of invented ones.

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

backend/app/rag/adventure_retrieval.py

@@ -0,0 +1,58 @@
+import asyncio
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.adventure_chunk import AdventureChunk
+from app.rag.embeddings import embed_query
+
+DEFAULT_TOP_K = 5
+# Same calibration as the rulebook RAG (app/rag/retrieval.py) — see there for how these were chosen.
+MAX_DISTANCE = 0.6
+MAX_BLOCK_CHARS = 4000
+
+
+async def build_adventure_rag_block(
+    session: AsyncSession, game_id: uuid.UUID, query: str, top_k: int = DEFAULT_TOP_K
+) -> str:
+    # Chunk 0 (the adventure's opening) is always included alongside the similarity search:
+    # on the very first turns the player's message is often just answering Session-0 questions
+    # and won't necessarily be similar to the adventure's actual hook, so pure similarity search
+    # could miss the beginning entirely and leave the DM to improvise instead of following it.
+    opening = (
+        await session.execute(
+            select(AdventureChunk)
+            .where(AdventureChunk.game_id == game_id, AdventureChunk.chunk_index == 0)
+        )
+    ).scalar_one_or_none()
+
+    query_embedding = await asyncio.to_thread(embed_query, query)
+    distance = AdventureChunk.embedding.cosine_distance(query_embedding)
+    rows = (
+        await session.execute(
+            select(AdventureChunk, distance.label("distance"))
+            .where(AdventureChunk.game_id == game_id)
+            .order_by(distance)
+            .limit(top_k)
+        )
+    ).all()
+
+    relevant = [chunk for chunk, dist in rows if dist <= MAX_DISTANCE]
+    if opening is not None and opening.id not in {c.id for c in relevant}:
+        relevant.insert(0, opening)
+
+    if not relevant:
+        return ""
+
+    parts = [
+        "Auszüge aus dem vorgegebenen Abenteuer — halte dich strikt daran (siehe Systemanweisung):"
+    ]
+    used_chars = 0
+    for chunk in relevant:
+        entry = f"\n{chunk.content}"
+        if used_chars + len(entry) > MAX_BLOCK_CHARS and used_chars > 0:
+            break
+        parts.append(entry)
+        used_chars += len(entry)
+    return "\n".join(parts)