Tech debt: allow more database backends

Tracked in `roadmap.md` *Tech debts* as: **"allow more db backends"**

Today the database layer is PostgreSQL-only: `pgxpool` wrapped as a `database/sql` handle for Bun (`templates/backend/internal/database/database.go`), with Bun's own migrator on top.

Related: the **Transactor** row in *Published Contract Candidates* is marked *Undecided* precisely because it hangs on this — the transaction-boundary port is genuinely generic across Bun, pgx and sqlc, but it fails the "someone outside would write one" bar unless multi-backend support actually materialises. Revisit that verdict here.

Start with roadmap.md and templates/backend/internal/database/database.go to understand the current PostgreSQL-only setup, including pgxpool, database/sql, Bun, and migrations. Define which additional backends and compatibility expectations are in scope before changing the database layer. Done means multi-backend support is implemented and the Transactor contract can be revisited with evidence that an external user could adopt it.

Written by the indexing model from the issue text.