core, desktop: Db::conn everywhere, Change::default, notes_where, and shared row mappings

From the audit (#5179, core and desktop half).

- Every `db.0.lock().map_err(|e| e.to_string())?` (about 50 sites in core and
  the desktop) is now `db.conn()?`. The few sites that deliberately handle
  a poisoned lock differently, and the tests, keep their own spelling.
- push::Change derives Default, so its four constructors name only the
  fields they set.
- store: list_notes, reminders, titles and search share notes_where (ids
  from a query, each loaded through load_note). Labels share
  LABEL_SELECT/label_row, and saved filters share
  SAVED_FILTER_SELECT/saved_filter_row.

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

core/src/sync/push.rs

@@ -64,7 +64,7 @@ impl PushSummary {
 /// One entry in the `changes` array. Notes and labels share the envelope; serde skips
 /// the fields that don't apply, so the server sees exactly the shape docs/sync.md
 /// describes for each entity.
-#[derive(Debug, Serialize)]
+#[derive(Debug, Default, Serialize)]
 pub struct Change {
     pub entity: &'static str,
     pub id: String,
@@ -110,18 +110,7 @@ impl Change {
             id,
             op: "delete",
             edited_at,
-            body: None,
-            color: None,
-            pinned: None,
-            archived: None,
-            trashed: None,
-            remind_at: None,
-            recurrence: None,
-            position: None,
-            label_ids: None,
-            created_at: None,
-            name: None,
-            state_at: None,
+            ..Default::default()
         }
     }
 }
@@ -237,16 +226,7 @@ fn collect_labels(conn: &Connection, out: &mut Vec<Change>, limit: usize) -> rus
             name: Some(r.get(1)?),
             color: Some(r.get(2)?),
             edited_at: r.get(3)?,
-            body: None,
-            pinned: None,
-            archived: None,
-            trashed: None,
-            remind_at: None,
-            recurrence: None,
-            position: None,
-            label_ids: None,
-            created_at: None,
-            state_at: None,
+            ..Default::default()
         })
     })?;
     for row in rows {
@@ -342,17 +322,11 @@ fn note_change(conn: &Connection, id: &str, shared_state: bool) -> rusqlite::Res
             op: "upsert",
             edited_at: row.updated_at,
             body: (row.permission == "edit").then_some(row.body),
-            color: None,
             pinned: own.then_some(row.pinned),
             archived: own.then_some(row.archived),
-            trashed: None,
-            remind_at: None,
-            recurrence: None,
             position: own.then_some(row.position),
-            label_ids: None,
-            created_at: None,
-            name: None,
             state_at,
+            ..Default::default()
         });
     }
 
@@ -374,8 +348,6 @@ fn note_change(conn: &Connection, id: &str, shared_state: bool) -> rusqlite::Res
         // server's last-write-wins comparison runs against.
         edited_at: row.updated_at,
         body: Some(row.body),
-        // A note has no colour to send. See the field on `Change`.
-        color: None,
         pinned: Some(row.pinned),
         archived: Some(row.archived),
         trashed: Some(row.trashed),
@@ -384,8 +356,7 @@ fn note_change(conn: &Connection, id: &str, shared_state: bool) -> rusqlite::Res
         position: Some(row.position),
         label_ids: Some(label_ids),
         created_at: Some(row.created_at),
-        name: None,
-        state_at: None,
+        ..Default::default()
     })
 }
 
@@ -610,7 +581,7 @@ async fn upload_pending(
     token: &str,
 ) -> Result<PushSummary, String> {
     let wanted = {
-        let conn = db.0.lock().map_err(|e| e.to_string())?;
+        let conn = db.conn()?;
         pending_uploads(&conn).map_err(|e| e.to_string())?
     };
     let mut summary = PushSummary::default();
@@ -636,7 +607,7 @@ async fn upload_pending(
             )),
         };
         {
-            let conn = db.0.lock().map_err(|e| e.to_string())?;
+            let conn = db.conn()?;
             settle_upload(&conn, &upload.id, &result).map_err(|e| e.to_string())?;
         }
         match result {
@@ -669,7 +640,7 @@ pub async fn run(
 
     loop {
         let batch = {
-            let conn = db.0.lock().map_err(|e| e.to_string())?;
+            let conn = db.conn()?;
             collect(&conn, BATCH, accepts).map_err(|e| e.to_string())?
         };
         if batch.is_empty() {
@@ -680,7 +651,7 @@ pub async fn run(
         let results = parse_results(&raw)?;
 
         let applied = {
-            let conn = db.0.lock().map_err(|e| e.to_string())?;
+            let conn = db.conn()?;
             apply_results(&conn, &batch, &results).map_err(|e| e.to_string())?
         };
         // Everything rejected clears nothing, so the same batch would be collected