Investigate generating Mermaid ER diagrams from database schema

We want ER diagrams that stay in sync with the schema rather than hand-maintained. Investigate whether diagrams can be generated directly from the running MariaDB instance or .sql files.

- [ ] Evaluate [mermerd](https://github.com/KarnerTh/mermerd). Connect to the dev database and assess output quality (table coverage, foreign key rendering, readability)
- [ ] Identify alternatives (e.g. `schemacrawler`, `tbls`, `mysql-to-mermaid`, or a custom SQL query against `information_schema`)
- [ ] Assess whether the chosen tool can run in CI to keep diagrams up to date

We want to balance the benefit of viewable schema to lead to simplification and stability vs the cost of additional dependencies.