feat: add CoinGecko API key support with env-based attribution and docs updates

.env.example

@@ -1,3 +1,12 @@
-# No API keys are required for the default setup.
+# Optional: CoinGecko API key (server-side only).
+# Preferred:
+# COINGECKO_PRO_API_KEY=
+# or
+# COINGECKO_DEMO_API_KEY=
+#
+# Fallback generic option:
+# COINGECKO_API_KEY=
+# COINGECKO_API_KEY_TYPE=demo  # demo | pro
+#
 # Optional: override the internal API route base when deploying behind a proxy.
 # NEXT_PUBLIC_API_BASE_URL=