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/plugin.py

@@ -0,0 +1,62 @@
+"""Slides plugin main class.
+
+Display/service plugin: contributes a blueprint only (no model, no AssetType,
+no nav). Serves TV-dashboard slideshow images. Demonstrates the minimal plugin
+shape - a feature that is purely an API surface.
+"""
+
+import json
+import logging
+from pathlib import Path
+from typing import List, Dict, Optional, Type
+
+from flask import Flask, Blueprint
+
+from shopdb.plugins.base import BasePlugin, PluginMeta
+
+from .api import slides_bp
+
+logger = logging.getLogger(__name__)
+
+
+class SlidesPlugin(BasePlugin):
+    """Slides plugin - TV dashboard slideshow images."""
+
+    def __init__(self):
+        self._manifest = self._load_manifest()
+
+    def _load_manifest(self) -> Dict:
+        """Load plugin manifest from JSON file."""
+        manifest_path = Path(__file__).parent / 'manifest.json'
+        if manifest_path.exists():
+            with open(manifest_path, 'r') as f:
+                return json.load(f)
+        return {}
+
+    @property
+    def meta(self) -> PluginMeta:
+        """Return plugin metadata."""
+        return PluginMeta(
+            name=self._manifest.get('name', 'slides'),
+            version=self._manifest.get('version', '1.0.0'),
+            description=self._manifest.get(
+                'description',
+                'TV dashboard slideshow images'
+            ),
+            author=self._manifest.get('author', 'ShopDB Team'),
+            dependencies=self._manifest.get('dependencies', []),
+            core_version=self._manifest.get('core_version', '>=0.1.0,<1.0.0'),
+            api_prefix=self._manifest.get('api_prefix', '/api/slides'),
+        )
+
+    def get_blueprint(self) -> Optional[Blueprint]:
+        """Return Flask Blueprint with API routes."""
+        return slides_bp
+
+    def get_models(self) -> List[Type]:
+        """No models - slides are read from the filesystem."""
+        return []
+
+    def init_app(self, app: Flask, db_instance) -> None:
+        """Initialize plugin with Flask app."""
+        logger.info(f"Slides plugin initialized (v{self.meta.version})")