feat(deploy): single-container Docker packaging (nginx + Flask + prebuilt SPA)

Add self-contained Docker deployment for EasyPanel / docker-compose:

- Dockerfile: python:3.11-slim + nginx. Serves the PREBUILT Vue SPA
  (frontend/dist, committed) via nginx and reverse-proxies /api to the Flask
  backend on 127.0.0.1:5000. No node/npm in the image (avoids Vite/npm
  flakiness in Docker builds). VOLUME /app/backend/data for persistence
  (factor vintages, dividend ledger, forward runs, prices). HEALTHCHECK hits
  /api/v1/dashboard/summary. Exposes :80.
- deploy/nginx.conf: SPA fallback + /api proxy to 127.0.0.1:5000 + gzip.
- deploy/entrypoint.sh: starts Flask (HOST=0.0.0.0:5000) + nginx foreground,
  forwards signals.
- docker-compose.yml: single service 'set50', port 8080:80, mounts
  ./backend/data for persistence, healthcheck.
- .dockerignore excludes .git/.venv/backend/data/node_modules/docs.
- .gitignore now tracks frontend/dist/ (prebuilt bundle required by the
  image); backend/data stays untracked.
- Backend runtime verified: test_client GET /api/v1/dashboard/summary=200
  (the HEALTHCHECK path). entrypoint sh -n, compose YAML parse, and all
  Dockerfile-referenced files exist. NOTE: no local docker here, so the
  image itself was not built — that happens on EasyPanel.

docker-compose.yml

@@ -0,0 +1,26 @@
+# Local/self-hosted single-container run for the SET50 alternative-data platform.
+# Production (EasyPanel) builds from the Dockerfile directly; this compose file
+# adds a persistent volume so factor vintages, the dividend ledger, forward runs
+# and prices survive container recreate.
+services:
+  set50:
+    build: .
+    image: set50-alternative-data:latest
+    ports:
+      - "8080:80"
+    environment:
+      # bind backend on all interfaces inside the container (nginx proxies /api)
+      HOST: "0.0.0.0"
+      PORT: "5000"
+      PAPER_BIND_HOST: "0.0.0.0"
+      # optional: make the dividend refresh faster for demos ()
+      # DIVIDEND_REFRESH_COOLDOWN_SECONDS: "300"
+    volumes:
+      - ./backend/data:/app/backend/data
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "python", "-c", "import urllib.request,sys; sys.exit(0 if urllib.request.urlopen('http://127.0.0.1/api/v1/dashboard/summary', timeout=4).status==200 else 1)"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+      start_period: 10s