Fix unpermitted params notices from the backend

[kevin-atnos]

This fix unpermitted params notices on Structures creation, PIA creation and Answer creation when pia is used with pia-back (client/server mode).

The fix filters out id, created_at, and updated_at from being sent to the server during updates. These fields are server-managed and shouldn't be included in API requests.

• The update() method in ApplicationDb was sending ALL properties from the entry object to the Rails backend
• Rails correctly flagged id, created_at, updated_at as unpermitted parameters since they shouldn't be modifiable via the API

The fix:

• Added an excludedFields array containing ['id', 'created_at', 'updated_at']
• These fields are now skipped when building the FormData for API updates
• Also added user_pias to the excluded fields. This field contains relational data (the association between users and PIAs) that shouldn't be sent to the server during updates - it's managed by the backend through a separate join table.

Answer management:
updated both:
- Backend (see PR on pia-back):
- Changed answer_params to permit :data as a string
- Added processed_answer_params that parses the JSON string (same pattern as structures_controller)
- Frontend (pia/src/app/services/answer.service.ts):
- Serialize data as JSON string instead of nested form fields
This follows the same pattern used for structures.