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>

core/src/local/mod.rs

@@ -84,7 +84,7 @@ pub fn open_in_memory() -> rusqlite::Result<Db> {
 
 /// A one-line count summary of the store, for the startup log.
 pub fn summary(db: &Db) -> String {
-    let conn = match db.0.lock() {
+    let conn = match db.conn() {
         Ok(c) => c,
         Err(_) => return "counts unavailable (lock poisoned)".to_string(),
     };