Extract slideshow into a plugin (blueprint-only display plugin)

Second core feature pulled into a plugin. Slides is the minimal plugin shape:
a blueprint only - no model, no AssetType, no nav (it is purely an API surface
consumed by the TV dashboard).

- plugins/slides/: manifest (api_prefix /api/slides, no deps), api/ (moved
  blueprint, contract-pure imports via shopdb.api, same /api/slides route so the
  TV dashboard is unchanged), plugin.py (get_blueprint, get_models -> []).
- Fixed a latent bug in the move: error path used ErrorCodes.SERVER_ERROR which
  does not exist -> ErrorCodes.INTERNAL_ERROR.
- De-cored: deleted shopdb/core/api/slides.py, removed from CORE_BLUEPRINT_NAMES
  + core/api/__init__ import/__all__. Registered in instance/plugins.json.

Pinned with a characterization test first; passes unchanged against the plugin
blueprint. 174 tests pass, naming green, app boots 8 bundled plugins, /api/slides
verified live.

Note: the shopfloor dashboard is NOT extractable - it is a frontend-only Vue view
aggregating the businessunits + notifications APIs, with no backend to move and no
frontend plugin system.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/slides/api/__init__.py

@@ -0,0 +1,5 @@
+"""Slides plugin API."""
+
+from .routes import slides_bp
+
+__all__ = ['slides_bp']