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/packages/harness/deerflow/persistence/workspace/sql.py

@@ -32,7 +32,7 @@ _SLUG_MAX_LEN = 32
 
 # slug 黑名单（应用层校验，不写 DB constraint）。包含 ADR-007 §4 保留 slug
 # + 路径 + Next.js 保留 + 业务保留词。
-_SLUG_BLACKLIST = frozenset(
+SLUG_BLACKLIST = frozenset(
     {
         "admin",
         "api",
@@ -77,7 +77,7 @@ def _validate_slug(slug: str) -> None:
         raise WorkspaceValidationError(f"slug length must be between {_SLUG_MIN_LEN} and {_SLUG_MAX_LEN}, got {len(slug)}")
     if not _SLUG_PATTERN.fullmatch(slug):
         raise WorkspaceValidationError(f"slug {slug!r} does not match required pattern ^[a-z0-9](-?[a-z0-9])*$")
-    if slug in _SLUG_BLACKLIST:
+    if slug in SLUG_BLACKLIST:
         raise WorkspaceValidationError(f"slug {slug!r} is reserved")