[Feature] Add message persistence layer (DB)

Create database storage for forwarded messages and metadata.\n\nAcceptance Criteria:\n- [ ] Define message storage schema\n- [ ] Implement repository abstraction\n- [ ] Persist forwarded messages on receive\n- [ ] Add config for DB connection\n

No files or tests are named. Start by inspecting the repository structure, configuration entry points, and the code path that receives forwarded messages. Define the storage schema, repository boundary, persistence behavior, and database configuration; done means all four acceptance criteria are implemented and verified.

Written by the indexing model from the issue text.