## PR: Certificate Pre-Validation for Document Signing
### Problem
When a participant uploaded a certificate to sign a document, there was
no validation at submission time. If the certificate had the wrong
password, was expired, or was incompatible with the signing algorithm,
the error only surfaced during **finalization** — potentially days
later, after all other participants had signed. At that point the
session is stuck with no way to recover.
Additionally, `buildKeystore` in the finalization service only
recognised `"P12"` as a cert type, causing a `400 Invalid certificate
type: PKCS12` error when the **owner** signed using the standard
`PKCS12` identifier.
---
### What this PR does
#### Backend — Certificate pre-validation service
Adds `CertificateSubmissionValidator`, which validates a keystore before
it is stored by:
1. Loading the keystore with the provided password (catches wrong
password / corrupt file)
2. Checking the certificate's validity dates (catches expired and
not-yet-valid certs)
3. Test-signing a blank PDF using the same `PdfSigningService` code path
as finalization (catches algorithm incompatibilities)
This runs on both the participant submission endpoint
(`WorkflowParticipantController`) and the owner signing endpoint
(`SigningSessionController`), so both flows are protected.
#### Backend — Bug fix
`SigningFinalizationService.buildKeystore` now accepts `"PKCS12"` and
`"PFX"` as aliases for `"P12"`, consistent with how the validator
already handles them. This fixes a `400` error when the owner signed
using the `PKCS12` cert type.
#### Frontend — Real-time validation feedback
`ParticipantView` gains a debounced validation call (600ms) triggered
whenever the cert file or password changes. The UI shows:
- A spinner while validating
- Green "Certificate valid until [date] · [subject name]" on success
- Red error message on failure (wrong password, expired, not yet valid)
- The submit button is disabled while validation is in flight
#### Tests — Three layers
| Layer | File | Coverage |
|---|---|---|
| Service unit | `CertificateSubmissionValidatorTest` | 11 tests — valid
P12/JKS, wrong password, corrupt bytes, expired, not-yet-valid, signing
failure, cert type aliases |
| Controller unit | `WorkflowParticipantValidateCertificateTest` | 4
tests — valid cert, invalid cert, missing file, invalid token |
| Controller integration | `CertificateValidationIntegrationTest` | 6
tests — real `.p12`/`.jks` files through the full controller → validator
stack |
| Frontend E2E | `CertificateValidationE2E.spec.ts` | 7 Playwright tests
— all feedback states, button behaviour, SERVER type bypass |
#### CI
- **PR**: Playwright runs on chromium when frontend files change (~2-3
min)
- **Nightly / on-demand**: All three browsers (chromium, firefox,
webkit) at 2 AM UTC, also manually triggerable via `workflow_dispatch`
