feat(auth): decode_token rejects legacy 4-field JWTs as WORKSPACE_MISSING

After PR4 every JWT must carry wid (workspace_id). decode_token now
returns the new TokenError.WORKSPACE_MISSING when the signature is
valid but the payload lacks wid; expired tokens still report EXPIRED
first so /auth/refresh logic stays correct. AuthErrorCode gains a
matching WORKSPACE_REQUIRED for middleware to surface to clients.

Updates 13 existing test sites that issued tokens without wid to pass
workspace_id="ws-test" + role="owner", reflecting the new contract.

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

backend/app/gateway/auth/errors.py

@@ -21,6 +21,7 @@ class AuthErrorCode(StrEnum):
     PROVIDER_NOT_FOUND = "provider_not_found"
     NOT_AUTHENTICATED = "not_authenticated"
     SYSTEM_ALREADY_INITIALIZED = "system_already_initialized"
+    WORKSPACE_REQUIRED = "workspace_required"
 
 
 class TokenError(StrEnum):
@@ -29,6 +30,7 @@ class TokenError(StrEnum):
     EXPIRED = "expired"
     INVALID_SIGNATURE = "invalid_signature"
     MALFORMED = "malformed"
+    WORKSPACE_MISSING = "workspace_missing"
 
 
 class AuthErrorResponse(BaseModel):
@@ -42,4 +44,6 @@ def token_error_to_code(err: TokenError) -> AuthErrorCode:
     """Map TokenError to AuthErrorCode — single source of truth."""
     if err == TokenError.EXPIRED:
         return AuthErrorCode.TOKEN_EXPIRED
+    if err == TokenError.WORKSPACE_MISSING:
+        return AuthErrorCode.WORKSPACE_REQUIRED
     return AuthErrorCode.TOKEN_INVALID