chore: expose Flask server for gunicorn, add gunicorn to requirements

- app.py: server = app.server (required for production WSGI deployment)
- requirements.txt: add gunicorn>=21.2.0

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

app.py

@@ -1478,6 +1478,7 @@ app = dash.Dash(
     title="Fendt Sentinel",
     long_callback_manager=_long_callback_manager,
 )
+server = app.server
 
 # ── Flask-Caching — memoize expensive DB + graph calls ────────
 # SimpleCache keeps results in-process (no Redis needed for single-worker Dash).

requirements.txt

@@ -100,6 +100,9 @@ diskcache>=5.6.0
 multiprocess>=0.70.16
 # ── Server-side caching (tab navigation + DB query memoization) ──────────────
 flask-caching>=2.1.0
+
+# ── Production Web Server ────────────────────────────────────────────────────
+gunicorn>=21.2.0
 # ── Background scheduler (Scout 6-hour cycle + heartbeat) ───────────────────
 APScheduler>=3.10.0,<4.0
 # ── RSS feed parsing (Scout scraper) ─────────────────────────────────────────