feat(mcp): persist and display marketplace README

Capture the README markdown from LangBot Space when installing an MCP
server and store it on the mcp_servers record (new readme column +
alembic migration 0004). The detail page can then render docs offline,
independent of the server's runtime/connection state.

src/langbot/pkg/persistence/alembic/versions/0004_add_mcp_readme.py

@@ -0,0 +1,34 @@
+"""add readme column to mcp_servers
+
+Revision ID: 0004_add_mcp_readme
+Revises: 0003_add_rerank_models
+Create Date: 2026-06-06
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = '0004_add_mcp_readme'
+down_revision = '0003_add_rerank_models'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Add ``readme`` to mcp_servers if the table exists and the column is missing
+    # (the table may have been created by create_all() with the column already
+    # present on fresh installs, so guard against duplicate-add).
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    if 'mcp_servers' not in inspector.get_table_names():
+        return
+    columns = {col['name'] for col in inspector.get_columns('mcp_servers')}
+    if 'readme' not in columns:
+        op.add_column(
+            'mcp_servers',
+            sa.Column('readme', sa.Text(), nullable=False, server_default=''),
+        )
+
+
+def downgrade() -> None:
+    op.drop_column('mcp_servers', 'readme')