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>

frontend/src/pages/GameSession.tsx

@@ -163,7 +163,9 @@ export default function GameSession() {
     <div className="mx-auto flex h-[calc(100vh-56px)] max-w-6xl flex-col px-4 py-4">
       <div className="mb-3 flex items-center justify-between">
         <div>
-          <h1 className="text-xl font-bold text-amber-400">{game.name}</h1>
+          <h1 className="text-xl font-bold text-amber-400">
+            {game.name} {game.has_adventure && <span className="text-sm font-normal">📖</span>}
+          </h1>
           <p className="text-xs text-slate-500">{game.description}</p>
         </div>
         <div className="flex gap-2">