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/coredata/migrations/20260424T112802Z.sql

@@ -0,0 +1,16 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE cookie_categories ADD COLUMN posthog_consent BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE cookie_categories ALTER COLUMN posthog_consent DROP DEFAULT;