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/alembic/versions/f1a2b3c4d5e6_add_adventure_chunks.py

@@ -0,0 +1,45 @@
+"""add adventure_text on games and adventure_chunks table (per-game RAG)
+
+Revision ID: f1a2b3c4d5e6
+Revises: e5f7a1c3b982
+Create Date: 2026-09-02 10:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from pgvector.sqlalchemy import Vector
+
+# revision identifiers, used by Alembic.
+revision: str = 'f1a2b3c4d5e6'
+down_revision: Union[str, None] = 'e5f7a1c3b982'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'games',
+        sa.Column('adventure_text', sa.Text(), nullable=False, server_default=''),
+    )
+
+    op.create_table(
+        'adventure_chunks',
+        sa.Column('id', sa.UUID(), nullable=False),
+        sa.Column('game_id', sa.UUID(), nullable=False),
+        sa.Column('chunk_index', sa.Integer(), nullable=False),
+        sa.Column('content', sa.Text(), nullable=False),
+        sa.Column('embedding', Vector(384), nullable=False),
+        sa.ForeignKeyConstraint(['game_id'], ['games.id'], ondelete='CASCADE'),
+        sa.PrimaryKeyConstraint('id'),
+    )
+    op.create_index(
+        op.f('ix_adventure_chunks_game_id'), 'adventure_chunks', ['game_id'], unique=False
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(op.f('ix_adventure_chunks_game_id'), table_name='adventure_chunks')
+    op.drop_table('adventure_chunks')
+    op.drop_column('games', 'adventure_text')