Some database operations are const-safe

src/dds/build/deps.cpp

     }
 }
 
-deps_rebuild_info dds::get_rebuild_info(database& db, path_ref output_path) {
+deps_rebuild_info dds::get_rebuild_info(const database& db, path_ref output_path) {
     std::unique_lock lk{db.mutex()};
     auto             cmd_ = db.command_of(output_path);
     if (!cmd_) {

src/dds/build/deps.hpp

     std::string           previous_command_output;
 };
 
-deps_rebuild_info get_rebuild_info(database& db, path_ref output_path);
+deps_rebuild_info get_rebuild_info(const database& db, path_ref output_path);
 
 }  // namespace dds

src/dds/db/database.cpp

     sqlite3::exec(st, std::forward_as_tuple(fs::weakly_canonical(file).string()));
 }
 
-std::optional<std::vector<input_file_info>> database::inputs_of(path_ref file_) {
+std::optional<std::vector<input_file_info>> database::inputs_of(path_ref file_) const {
     auto  file = fs::weakly_canonical(file_);
     auto& st   = _stmt_cache(R"(
         WITH file AS (
     return ret;
 }
 
-std::optional<command_info> database::command_of(path_ref file_) {
+std::optional<command_info> database::command_of(path_ref file_) const {
     auto  file = fs::weakly_canonical(file_);
     auto& st   = _stmt_cache(R"(
         WITH file AS (

src/dds/db/database.hpp

 };
 
 class database {
-    neo::sqlite3::database        _db;
-    neo::sqlite3::statement_cache _stmt_cache{_db};
-    mutable std::shared_mutex     _mutex;
+    neo::sqlite3::database                _db;
+    mutable neo::sqlite3::statement_cache _stmt_cache{_db};
+    mutable std::shared_mutex             _mutex;
 
     explicit database(neo::sqlite3::database db);
     database(const database&) = delete;
     void store_file_command(path_ref file, const command_info& cmd);
     void forget_inputs_of(path_ref file);
 
-    std::optional<std::vector<input_file_info>> inputs_of(path_ref file);
-    std::optional<command_info>                command_of(path_ref file);
+    std::optional<std::vector<input_file_info>> inputs_of(path_ref file) const;
+    std::optional<command_info>                 command_of(path_ref file) const;
 };
 
 }  // namespace dds