Constrain PostHog consent to one normal category per banner

Add a partial unique index ensuring only one category per banner can
have posthog_consent enabled. Default it to the analytics category on
banner creation, clear the previous mapping before setting a new one,
and restrict the toggle to NORMAL categories in both the service layer
and the console UI.

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

pkg/coredata/migrations/20260424T122219Z.sql

@@ -0,0 +1,17 @@
+-- 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.
+
+CREATE UNIQUE INDEX idx_cookie_categories_unique_posthog_per_banner
+  ON cookie_categories (cookie_banner_id)
+  WHERE posthog_consent = true;