feat: web UI + SQLite storage + FastAPI backend

- app.py: unified app – ntfy subscriber runs as background thread,
  FastAPI serves REST API (/api/messages) and static frontend
- static/index.html: terminal-style notification table with
  sortable columns, drag & drop column reorder, column visibility
  toggle (localStorage), text search, forwarded-only filter,
  auto-refresh every 30s
- All ntfy fields stored: id, received_at, ntfy_time, topic,
  title, message, priority, tags, forwarded
- /data volume for persistent SQLite DB
- Port 8000 exposed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

portainer-stack.yml

@@ -3,5 +3,17 @@ services:
     image: git.albert-zangerl.com/al/logwatch:latest
     container_name: logwatch
     restart: unless-stopped
-    env_file:
-      - stack.env
+    ports:
+      - "8000:8000"
+    volumes:
+      - logwatch_data:/data
+    environment:
+      - SOURCE_URL=https://ntfy.albert-zangerl.com/albert
+      - TARGET_URL=https://ntfy.albert-zangerl.com/wichtig
+      # - TARGET_TOKEN=
+      - LOG_LEVEL=INFO
+      - RECONNECT_DELAY=10
+      - DB_PATH=/data/logwatch.db
+
+volumes:
+  logwatch_data: