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/calendar_dao.rs

@@ -274,7 +274,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
                 source_file: event.source_file,
             })
         })
-        .map_err(|_| DbError::new(DbErrorKind::InsertError))
+        .map_err(|e| DbError::log(DbErrorKind::InsertError, e))
     }
 
     fn store_events_batch(
@@ -348,7 +348,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
 
             Ok(inserted)
         })
-        .map_err(|_| DbError::new(DbErrorKind::InsertError))
+        .map_err(|e| DbError::log(DbErrorKind::InsertError, e))
     }
 
     fn find_events_in_range(
@@ -373,7 +373,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
             .map(|rows| rows.into_iter().map(|r| r.to_calendar_event()).collect())
             .map_err(|e| anyhow::anyhow!("Query error: {:?}", e))
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn find_similar_events(
@@ -429,7 +429,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
 
             Ok(scored_events.into_iter().take(limit).map(|(_, event)| event).collect())
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn find_relevant_events_hybrid(
@@ -500,7 +500,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
                 Ok(events_in_range.into_iter().take(limit).map(|r| r.to_calendar_event()).collect())
             }
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn event_exists(
@@ -528,7 +528,7 @@ impl CalendarEventDao for SqliteCalendarEventDao {
 
             Ok(result.count > 0)
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn get_event_count(&mut self, context: &opentelemetry::Context) -> Result<i64, DbError> {
@@ -551,6 +551,6 @@ impl CalendarEventDao for SqliteCalendarEventDao {
 
             Ok(result.count)
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 }