Add PostHog consent integration and extract integration plugin system

Add PostHog opt-in/opt-out consent support mirroring the existing Google
Consent Mode integration: database column, GraphQL field, console UI
toggle, and client-side posthog-js calls.

Extract both GCM and PostHog logic from CookieBannerClient into a
ConsentIntegration plugin interface so future integrations can be added
without modifying the client core.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/server/api/console/v1/cookie_banner_resolvers.go

@@ -472,6 +472,7 @@ func (r *mutationResolver) UpdateCookieCategory(ctx context.Context, input types
 			Slug:             input.Slug,
 			Description:      input.Description,
 			GCMConsentTypes:  gcmConsentTypes,
+			PostHogConsent:   input.PosthogConsent,
 		},
 	)
 	if err != nil {