feat: add ai_gateway_keys table and related RBAC (#25563)

Adds table to store keys that AI Gateway standalone replicas will use
to authenticate into Coderd.
Also adds RBAC and audit boilerplate.

coderd/database/migrations/testdata/fixtures/000514_ai_gateway_keys.up.sql

@@ -0,0 +1,15 @@
+INSERT INTO ai_gateway_keys (
+	id,
+	created_at,
+	name,
+	secret_prefix,
+	hashed_secret,
+	last_used_at
+) VALUES (
+	'8b6f0a82-9a3a-4d2e-8c0c-2c9c9b9b1a01',
+	'2026-05-21 00:00:00+00',
+	'example-key',
+	'cdr_1234567',
+	'\x00'::bytea,
+	NULL
+);