fix(database): align UUID defaults and add draft uniqueness constraint

- Create unique index for message_draft when thread_id is NULL
- Drop default values from ID columns across multiple message-related tables
- Ensure imks-managed IDs align with application-generated UUID v7 values
- Make top-level drafts unique per channel and user combination

migrate/006_fix_uuid_defaults_and_draft_null.sql

@@ -0,0 +1,32 @@
+-- Align imks-managed IDs with application-generated UUID v7 values and
+-- make top-level drafts unique when thread_id is NULL.
+
+BEGIN;
+
+CREATE UNIQUE INDEX IF NOT EXISTS uq_message_draft_channel_user_no_thread
+    ON message_draft (channel_id, user_id)
+    WHERE thread_id IS NULL;
+
+ALTER TABLE message_attachment ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_embed ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_embed_field ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_poll ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_poll_option ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_poll_vote ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_pin ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_read_state ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_draft ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_edit ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_article ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_reaction ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_bookmark ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_mention ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_thread ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_thread_participant ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_notification ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_scheduled ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_sticker ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_forward ALTER COLUMN id DROP DEFAULT;
+ALTER TABLE message_component ALTER COLUMN id DROP DEFAULT;
+
+COMMIT;