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/search_dao.rs

@@ -227,7 +227,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
                 source_file: search.source_file,
             })
         })
-        .map_err(|_| DbError::new(DbErrorKind::InsertError))
+        .map_err(|e| DbError::log(DbErrorKind::InsertError, e))
     }
 
     fn store_searches_batch(
@@ -283,7 +283,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
 
             Ok(inserted)
         })
-        .map_err(|_| DbError::new(DbErrorKind::InsertError))
+        .map_err(|e| DbError::log(DbErrorKind::InsertError, e))
     }
 
     fn find_searches_in_range(
@@ -310,7 +310,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
             .map(|rows| rows.into_iter().map(|r| r.to_search_record()).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_searches(
@@ -372,7 +372,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
                 .map(|(_, search)| search)
                 .collect())
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn find_relevant_searches_hybrid(
@@ -459,7 +459,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
                     .collect())
             }
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn search_exists(
@@ -490,7 +490,7 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
 
             Ok(result.count > 0)
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 
     fn get_search_count(&mut self, context: &opentelemetry::Context) -> Result<i64, DbError> {
@@ -513,6 +513,6 @@ impl SearchHistoryDao for SqliteSearchHistoryDao {
 
             Ok(result.count)
         })
-        .map_err(|_| DbError::new(DbErrorKind::QueryError))
+        .map_err(|e| DbError::log(DbErrorKind::QueryError, e))
     }
 }