Every DB operation (queue lease/enqueue, chat_state get/set, link_cache
read/write) used to open a fresh connection — including the busy timeout
and WAL pragma — then close it, on every message, URL job and callback.
Replace with DbPool: a tiny pool (4 connections max, semaphore-bounded
concurrency for backpressure) whose with_conn() method runs the closure on
a pooled connection inside spawn_blocking. Steady-state cost of an
operation is a list pop + semaphore acquire instead of a connection open.
/clear_cache with no argument wipes the whole link_cache table;
with a post URL it removes that single entry (normalized via
site::cache_key so fxtwitter/mobile/photo variants collide with
the write-side key). Non-admins get 'Admin only.'. LinkCache gains
clear(Option<&str>) -> usize reporting removed rows.
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.
YoursFunny