Data groups: a provider's models read only their own group's data

Every connection is in a data group. Its models are handed, and can find,
only that group's memories, notes, skills, knowledge, reports and
personality -- by search and by id. A chat stays in the group it was
started in: switching its model, the endpoint fallback, the crowd, friends,
bases and the @ menu all stay inside it, and a chat whose model has moved
is refused rather than sent. A group may name its own embedder and image
reviewer. data.manage lets a person make personal groups, remap
connections for themselves and move their own records.

Also: a search no longer mixes two embedders of the same width.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lembas/db/models/__init__.py

@@ -36,6 +36,7 @@ from lembas.db.models.chat import (
     Message,
 )
 from lembas.db.models.connection import Connection, Model, model_groups
+from lembas.db.models.data_group import DEFAULT_GROUP, DataGroup, InDataGroup
 from lembas.db.models.image import ImageWorkflow
 from lembas.db.models.library import (
     AUTHOR_MODEL,
@@ -167,6 +168,9 @@ __all__ = [
     "CrowdMember",
     "Job",
     "Connection",
+    "DEFAULT_GROUP",
+    "DataGroup",
+    "InDataGroup",
     "CustomTool",
     "CHUNK_DOCUMENT",
     "CHUNK_KINDS",