Cover the media and tags service layers

**Files:** `rslib/src/media/service.rs` (109, 0%) + `rslib/src/tags/service.rs` (107, 0%) · **Target: ≥ 65% each**

Two small services, same setup pattern.

Scope — media:

- add/list/remove media routing to the correct handler and persisting.

Scope — tags:

- add/remove/rename/clear;
- find by tag; tag-name canonify.