refactor: provider descriptor registry — eliminate provider-specific hardcoding

Replace all if/else chains keyed on provider type strings with a
descriptor-driven architecture. Each provider type (immich, gitea,
planka, scheduler, nut, google_photos) has a descriptor in
frontend/src/lib/providers/ that declares config fields, event
tracking fields, collection metadata, validation, and hooks.

Components now use getDescriptor(type) and render dynamically.
Dashboard provider card shows provider name + type when global
filter is active. Grid-items derived from registry.

scripts/restart-backend.sh

@@ -15,7 +15,8 @@ fi
 # Start backend
 NOTIFY_BRIDGE_DATA_DIR=./test-data \
 NOTIFY_BRIDGE_SECRET_KEY=test-secret-key-minimum-32chars \
-nohup python -m uvicorn notify_bridge_server.main:app \
+PYTHON=$(command -v python3 2>/dev/null || command -v python 2>/dev/null || command -v py 2>/dev/null)
+nohup "$PYTHON" -m uvicorn notify_bridge_server.main:app \
   --host 0.0.0.0 --port 8420 > /dev/null 2>&1 &
 
 sleep 3