refactor(db): rename search_space_memberships table to workspace_memberships

Phase 1 (rename DB) commit 1g: rename the memberships table (no inbound
FKs reference it) and update the matching raw-SQL cleanup in the obsidian
plugin test. Relationship attribute names stay until Phase 2.

surfsense_backend/app/db.py

@@ -2102,7 +2102,7 @@ class SearchSpaceMembership(BaseModel, TimestampMixin):
     Each user can be a member of multiple search spaces with different roles.
     """
 
-    __tablename__ = "search_space_memberships"
+    __tablename__ = "workspace_memberships"
     __table_args__ = (
         UniqueConstraint(
             "user_id",

surfsense_backend/tests/integration/test_obsidian_plugin_routes.py

@@ -125,7 +125,7 @@ async def race_user_and_space(async_engine):
             {"uid": user_id},
         )
         await cleanup.execute(
-            text("DELETE FROM search_space_memberships WHERE workspace_id = :id"),
+            text("DELETE FROM workspace_memberships WHERE workspace_id = :id"),
             {"id": space_id},
         )
         await cleanup.execute(