Add updated-from entity snapshot to updated webhooks

Update webhook events now carry a top-level "updatedFrom" field
alongside "data", containing a full snapshot of the entity as it was
before the update. This lets subscribers diff old vs new state (for
example the prior membership role on user:updated) without tracking
prior state themselves. It is a complete snapshot with the same shape as
"data", not a partial diff, so consumers select whatever fields they
need. The field is omitted for non-update events.

The webhook_data table gains a nullable updated_from JSONB column, and
webhook.InsertUpdateData enqueues both snapshots; InsertData delegates to
it with a nil updatedFrom so non-update callers are unaffected. Each
*:updated emission site snapshots the entity right after load, before
mutation: obligation, third-party, user (org and SCIM flows), document,
document-version, and document-version-approval-quorum. The document
emit helpers gained an optional updatedFrom argument threaded through to
the payload.

For document-version-approval-quorum:updated the snapshot requires an
extra query, so it is now gated behind the same subscription-existence
check the emitter uses: when no subscriber is configured the load is
skipped entirely rather than running (and potentially failing the
approval) for an event nobody receives.

Add integration tests (against a real Postgres, skipped when none is
reachable) covering the updated_from round-trip, the SQL NULL behavior
when no snapshot is provided, and the no-op when no subscription matches,
plus a unit test asserting updatedFrom is omitted from the payload when
absent.

Document the new field in the probod and n8n changelogs and the n8n
README.

Signed-off-by: Sacha Al Himdani <sacha@probo.com>

pkg/coredata/webhook_data.go

@@ -39,6 +39,7 @@ type (
 		OrganizationID gid.GID          `db:"organization_id"`
 		EventType      WebhookEventType `db:"event_type"`
 		Data           json.RawMessage  `db:"data"`
+		UpdatedFrom    json.RawMessage  `db:"updated_from"`
 		CreatedAt      time.Time        `db:"created_at"`
 		ProcessedAt    *time.Time       `db:"processed_at"`
 	}
@@ -58,6 +59,7 @@ INSERT INTO webhook_data (
     organization_id,
     event_type,
     data,
+    updated_from,
     created_at
 )
 VALUES (
@@ -66,6 +68,7 @@ VALUES (
     @organization_id,
     @event_type,
     @data,
+    @updated_from,
     @created_at
 )
 `
@@ -76,6 +79,7 @@ VALUES (
 		"organization_id": w.OrganizationID,
 		"event_type":      w.EventType,
 		"data":            w.Data,
+		"updated_from":    w.UpdatedFrom,
 		"created_at":      w.CreatedAt,
 	}
 
@@ -97,6 +101,7 @@ SELECT
     organization_id,
     event_type,
     data,
+    updated_from,
     created_at,
     processed_at
 FROM webhook_data