feat: add chat

migrations/0049_group_send_policy.up.sql

@@ -0,0 +1,26 @@
+-- Who may post in a group.
+--
+-- 'everyone' keeps existing groups behaving exactly as they do today, which is
+-- why it is the default: this migration must not silently mute anyone.
+-- 'admins' makes the thread an announcement channel — owners and admins post,
+-- everyone else reads.
+--
+-- DMs and Kautilya threads ignore this: a two-party conversation has no
+-- administrators, and the AI thread has one participant.
+ALTER TABLE conversations
+    ADD COLUMN IF NOT EXISTS send_policy VARCHAR(8) NOT NULL DEFAULT 'everyone';
+
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM pg_constraint WHERE conname = 'conversations_send_policy_check'
+    ) THEN
+        ALTER TABLE conversations
+            ADD CONSTRAINT conversations_send_policy_check
+            CHECK (send_policy IN ('everyone', 'admins'));
+    END IF;
+END $$;
+
+-- "Who are this group's admins?" runs on every post in an admins-only thread.
+CREATE INDEX IF NOT EXISTS conversation_participants_role_idx
+    ON conversation_participants (conversation_id, role) WHERE left_at IS NULL;