fix: stop swallowing DB errors across the entire DAO layer

Every map_err(|_| DbError::new(...)) and map_err(|_| anyhow!("..."))
in the database layer was discarding the actual Diesel/SQLite error,
making failures impossible to diagnose from logs.

- Add DbError::log() that logs the source error before converting
- Replace all ~130 swallowed outer map_err closures with DbError::log
- Replace all ~47 swallowed inner anyhow closures to include the
  source error in the message

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/database/preview_dao.rs

@@ -96,7 +96,7 @@ impl PreviewDao for SqlitePreviewDao {
                 .map(|_| ())
                 .map_err(|e| anyhow::anyhow!("Insert error: {}", e))
         })
-        .map_err(|_| DbError::new(DbErrorKind::InsertError))
+        .map_err(|e| DbError::log(DbErrorKind::InsertError, e))
     }
 
     fn update_status(
@@ -126,7 +126,7 @@ impl PreviewDao for SqlitePreviewDao {
                 .map(|_| ())
                 .map_err(|e| anyhow::anyhow!("Update error: {}", e))
         })
-        .map_err(|_| DbError::new(DbErrorKind::UpdateError))
+        .map_err(|e| DbError::log(DbErrorKind::UpdateError, e))
     }
 
     fn get_preview(
@@ -148,7 +148,7 @@ impl PreviewDao for SqlitePreviewDao {
                 Err(e) => Err(anyhow::anyhow!("Query error: {}", e)),
             }
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn get_previews_batch(
@@ -170,7 +170,7 @@ impl PreviewDao for SqlitePreviewDao {
                 .load::<VideoPreviewClip>(connection.deref_mut())
                 .map_err(|e| anyhow::anyhow!("Query error: {}", e))
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn get_by_status(
@@ -188,7 +188,7 @@ impl PreviewDao for SqlitePreviewDao {
                 .load::<VideoPreviewClip>(connection.deref_mut())
                 .map_err(|e| anyhow::anyhow!("Query error: {}", e))
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 }