Every lock of the store goes through Db::conn

Db::conn is documented as the one way to take the lock, yet five production and
test sites reached past it with db.0.lock(): the startup summary, the desktop's
trash sweep and config_get, and tests in sharing and update. All five now call
conn().

DRY pass #2, batch 2, F8 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

desktop/src-tauri/src/commands/local.rs

@@ -25,7 +25,7 @@ pub fn config_get(db: State<'_, Db>) -> PublicConfig {
     // be enforced. A store error falls back to the default rather than failing the
     // call — the app must still boot.
     let fallback = retention::LOCAL_RETENTION_DAYS;
-    let retention_days = match db.0.lock() {
+    let retention_days = match db.conn() {
         Ok(conn) => state::effective_retention_days(&conn, fallback).unwrap_or(fallback),
         Err(_) => fallback,
     };