refactor: unify test dispatch with real NotificationDispatcher

- Route scheduled/memory test sends through the same NotificationDispatcher
  the watcher uses — identical template rendering, media handling, caching
- Add preview_url field to MediaAsset (transcoded mid-size), separate from
  thumbnail_url (small) and full_url (original). Dispatcher prefers preview_url
- Fix sendMediaGroup cache: extract file_ids from Telegram response and store
  via async_set_many so repeat sends use cached file_ids
- Parallelize asset downloads in _send_media_group with asyncio.gather
- Filter unprocessed assets (archived/trashed/offline/no-thumbhash) at album
  parse time in ImmichAlbumData.from_api_response
- Extract shared asset_to_media + collect_scheduled_assets into asset_utils.py
  (single source for test dispatch and future real scheduler)
- Respect tracking config filters: limit, asset_type, favorite_only, min_rating
- Random asset sampling for scheduled sends
- Memory mode: "On This Day" date filter (same month+day, previous year)
- Skip dispatch when no matching assets found
- Remove ~250 lines of duplicated send logic from notifier.py
- Fix restart-backend.sh: proper env var export, Python path resolution, error log

scripts/restart-backend.sh

@@ -2,7 +2,6 @@
 # Restart the backend dev server.
 # Usage: bash scripts/restart-backend.sh
 
-set -e
 cd "$(dirname "$0")/.."
 
 # Kill existing backend
@@ -12,12 +11,28 @@ if [ -n "$PID" ]; then
   sleep 1
 fi
 
+# Resolve python — py launcher needs absolute path for nohup on Windows
+if command -v python3 &>/dev/null; then
+  PYTHON=python3
+elif command -v python &>/dev/null; then
+  PYTHON=python
+else
+  PYTHON=$(py -3.13 -c "import sys; print(sys.executable)" 2>/dev/null \
+        || py -3 -c "import sys; print(sys.executable)")
+fi
+
 # Start backend
-NOTIFY_BRIDGE_DATA_DIR=./test-data \
-NOTIFY_BRIDGE_SECRET_KEY=test-secret-key-minimum-32chars \
-PYTHON=$(command -v python3 2>/dev/null || command -v python 2>/dev/null || echo "py -3.13")
+export NOTIFY_BRIDGE_DATA_DIR=./test-data
+export NOTIFY_BRIDGE_SECRET_KEY=test-secret-key-minimum-32chars
 nohup "$PYTHON" -m uvicorn notify_bridge_server.main:app \
-  --host 0.0.0.0 --port 8420 > /dev/null 2>&1 &
+  --host 0.0.0.0 --port 8420 > .backend.log 2>&1 &
 
 sleep 3
-curl -s http://localhost:8420/api/health
+if curl -sf http://localhost:8420/api/health; then
+  echo ""
+  echo "Backend started (PID $!)"
+else
+  echo "Backend failed to start. Log:"
+  tail -20 .backend.log
+  exit 1
+fi