Add self-hosted usage analytics: pageview tracking + admin stats dashboard

Records pageviews (path, referrer, browser/OS/device, IP, GeoIP country) via
a POST /api/track beacon into a new PageView table, and exposes aggregated
daily/breakdown stats behind a token-gated GET /api/admin/stats endpoint with
brute-force lockout. Frontend gets a /admin dashboard (charts + breakdown
tables) built on recharts, with a switchable per-day pageviews chart.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,25 @@
+# Copy to .env and fill in real values before `docker compose -f docker-compose.prod.yaml up`.
+# .env is gitignored -- never commit real secrets.
+
+POSTGRES_USER=bridzik
+POSTGRES_PASSWORD=bridzik
+POSTGRES_DB=bridzik
+
+# Comma-separated list of origins allowed to open a Socket.IO connection.
+# Must be the public URL(s) the frontend is served from -- never "*" in prod.
+CORS_ALLOWED_ORIGINS=https://bridzik.liptim.eu
+
+# Shared secret for the self-hosted /api/admin/* stats endpoints.
+ADMIN_TOKEN=tajneheslo
+
+# Fernet key encrypting Player.totp_secret at rest. Generate with:
+#   python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+# Keep it stable across restarts/redeploys -- losing it locks every account
+# out (their TOTP secret can no longer be decrypted).
+ENCRYPTION_KEY=
+
+# Optional: IP -> country for /api/track pageviews (see GEOIP_DB_PATH in
+# docker-compose.prod.yaml). Place a .mmdb file (GeoLite2-City/Country from a
+# MaxMind account, or a DB-IP/IP2Location Lite equivalent) at
+# ./geoip/GeoLite2-City.mmdb next to docker-compose.prod.yaml. Left missing ->
+# country is just recorded as "".