Fix: use domain_name (FQDN) in cell invite and conflict checks

The GET /api/cells/invite endpoint was returning domain='pic.ngo' instead
of the full FQDN 'test5.pic.ngo' because it read _identity.domain rather
than _identity.domain_name.

Apply the same domain_name preference (domain_name || domain) to:
- routes/cells.py get_cell_invite() — the invite shown to connecting cells
- routes/cells.py update_cell_permissions() — Corefile DNS regeneration
- cell_link_manager.py _check_invite_conflicts() — incoming domain collision check
- cell_link_manager.py exchange_invites() — own invite construction

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/cell_link_manager.py

@@ -426,7 +426,7 @@ class CellLinkManager:
             try:
                 from app import config_manager
                 identity = config_manager.configs.get('_identity', {})
-                own_domain = identity.get('domain', os.environ.get('CELL_DOMAIN', ''))
+                own_domain = identity.get('domain_name') or identity.get('domain', os.environ.get('CELL_DOMAIN', ''))
                 if own_domain and remote_domain == own_domain:
                     raise ValueError(
                         f"Domain {remote_domain!r} is the same as this cell's own domain — "
@@ -466,7 +466,7 @@ class CellLinkManager:
             identity = self._local_identity()
             from app import config_manager
             id_cfg = config_manager.configs.get('_identity', {})
-            own_domain = id_cfg.get('domain', os.environ.get('CELL_DOMAIN', 'cell'))
+            own_domain = id_cfg.get('domain_name') or id_cfg.get('domain', os.environ.get('CELL_DOMAIN', 'cell'))
             own_invite = self.generate_invite(identity['cell_name'], own_domain)
         except Exception as e:
             return {'ok': False, 'error': f'could not build own invite: {e}'}