refactor: share sqlite open/with_conn helpers in db.rs

Converge the duplicated open_db (open + busy_timeout) and the
spawn_blocking + expect ceremony that every table access repeated
into one db.rs module. ChatStore no longer creates the tasks table
(schema ownership: queue.rs owns tasks, state.rs chat_state,
link_cache.rs link_cache). No schema or behavior change - all
CREATE TABLE statements are byte-identical, IF NOT EXISTS stays
idempotent, so existing data/task_queue.db files need no migration.

crates/xmedia-bot/src/main.rs

@@ -8,6 +8,7 @@ use tokio::sync::watch;
 use x_media::site;
 
 mod config;
+mod db;
 mod handlers;
 mod link_cache;
 mod photo;