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/routes.py

@@ -0,0 +1,71 @@
+"""Slides API for the TV dashboard slideshow."""
+
+import os
+from flask import Blueprint, current_app
+
+from shopdb.api import success_response, error_response, ErrorCodes
+
+slides_bp = Blueprint('slides', __name__)
+
+# Valid image extensions
+VALID_EXTENSIONS = {'.jpg', '.jpeg', '.png', '.gif', '.bmp', '.webp'}
+
+
+@slides_bp.route('', methods=['GET'])
+def get_slides():
+    """Get list of slides for the TV dashboard.
+
+    Returns image files from the static/slides directory.
+    """
+    # Look for slides in static folder
+    static_folder = current_app.static_folder
+    if not static_folder:
+        static_folder = os.path.join(current_app.root_path, 'static')
+
+    slides_folder = os.path.join(static_folder, 'slides')
+
+    # Also check frontend public folder
+    frontend_slides = os.path.join(
+        os.path.dirname(os.path.dirname(os.path.dirname(current_app.root_path))),
+        'frontend', 'public', 'slides'
+    )
+
+    # Try multiple possible locations
+    possible_paths = [
+        slides_folder,
+        frontend_slides,
+        '/home/camp/projects/shopdb-flask/shopdb/static/slides',
+        '/home/camp/projects/shopdb-flask/frontend/public/slides',
+    ]
+
+    slides_path = None
+    for path in possible_paths:
+        if os.path.isdir(path):
+            slides_path = path
+            break
+
+    if not slides_path:
+        return success_response({
+            'slides': [],
+            'basepath': '/static/slides/',
+            'message': 'Slides folder not found'
+        })
+
+    # Get list of image files
+    slides = []
+    try:
+        for filename in sorted(os.listdir(slides_path)):
+            ext = os.path.splitext(filename)[1].lower()
+            if ext in VALID_EXTENSIONS:
+                slides.append({'filename': filename})
+    except Exception as e:
+        return error_response(
+            ErrorCodes.INTERNAL_ERROR,
+            f'Error reading slides: {str(e)}',
+            http_code=500
+        )
+
+    return success_response({
+        'slides': slides,
+        'basepath': '/static/slides/'
+    })