feat(auth): /initialize auto-creates default workspace + owner membership

initialize_admin now seeds a 1-person workspace immediately after the
admin user is created: WorkspaceRepository.create(name, slug, owner_id)
+ WorkspaceMembershipRepository.add(role='owner') + writes the new
workspace id back to users.default_workspace_id. The session JWT is
re-issued with wid + role='owner' so subsequent requests pass the
T4.7 workspace gate.

Mechanical pieces:
- SQLiteUserRepository row<->user mapping now includes
  default_workspace_id (sql update_user too) so the column persists.
- workspace.sql.SLUG_BLACKLIST is now public (was _SLUG_BLACKLIST) and
  the registration helper treats blacklisted slugs as "taken" so the
  walker steps past reserved names like "admin" instead of crashing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/gateway/auth/repositories/sqlite.py

@@ -46,6 +46,7 @@ class SQLiteUserRepository(UserRepository):
             oauth_id=row.oauth_id,
             needs_setup=row.needs_setup,
             token_version=row.token_version,
+            default_workspace_id=row.default_workspace_id,
         )
 
     @staticmethod
@@ -60,6 +61,7 @@ class SQLiteUserRepository(UserRepository):
             oauth_id=user.oauth_id,
             needs_setup=user.needs_setup,
             token_version=user.token_version,
+            default_workspace_id=user.default_workspace_id,
         )
 
     # ── CRUD ──────────────────────────────────────────────────────────
@@ -106,6 +108,7 @@ class SQLiteUserRepository(UserRepository):
             row.oauth_id = user.oauth_id
             row.needs_setup = user.needs_setup
             row.token_version = user.token_version
+            row.default_workspace_id = user.default_workspace_id
             await session.commit()
         return user