fix: Cannot edit custom fields imported via generic CSV importer

[Roshan931]

Fixes #365

Root cause

Imported custom fields lack field_type, so the edit form cannot render them

Changes

• Importers now store an explicit field_type ('text' or 'password' derived from the secret flag) for custom fields, and the edit-credential controller backfills a valid field_type for already-stored custom fields that miss it, so the edit form renders and allows editing their values.