Add rulebook RAG pipeline and LLM-driven game setup wizard

RAG: switch Postgres to pgvector, chunk and embed the three D&D
rulebooks locally via sentence-transformers, and retrieve relevant
excerpts per DM turn (query = latest player message) to ground the
system prompt. Retrieval runs off the event loop and is capped by a
relevance threshold and a max character budget so it can't blow up
context size or cost.

Game setup wizard: creating a game now opens a short chat where the
DM asks about genre, length, and the player's experience level, then
proposes a name and description via a tool call. The player can edit
both before creating the game. Stateless endpoint — the frontend
carries the conversation, no DB needed since the game doesn't exist
yet.

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

backend/app/alembic/versions/7c3f9a2e1d44_add_rulebook_chunks.py

@@ -0,0 +1,43 @@
+"""add rulebook_chunks (pgvector RAG)
+
+Revision ID: 7c3f9a2e1d44
+Revises: 48846ed4fe0e
+Create Date: 2026-08-31 19:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+from pgvector.sqlalchemy import Vector
+
+# revision identifiers, used by Alembic.
+revision: str = '7c3f9a2e1d44'
+down_revision: Union[str, None] = '48846ed4fe0e'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute("CREATE EXTENSION IF NOT EXISTS vector")
+
+    op.create_table(
+        'rulebook_chunks',
+        sa.Column('id', sa.UUID(), nullable=False),
+        sa.Column('source_document', sa.String(length=200), 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.Column('doc_metadata', postgresql.JSONB(astext_type=sa.Text()), nullable=False),
+        sa.PrimaryKeyConstraint('id'),
+    )
+    op.create_index(
+        op.f('ix_rulebook_chunks_source_document'), 'rulebook_chunks', ['source_document'], unique=False
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(op.f('ix_rulebook_chunks_source_document'), table_name='rulebook_chunks')
+    op.drop_table('rulebook_chunks')
+    op.execute("DROP EXTENSION IF EXISTS vector")