feat: add generative proxy with usage metrics

.env.example

@@ -0,0 +1,9 @@
+ADMIN_SECRET=replace-with-a-long-random-secret
+DB_PATH=./data/proxy.db
+MAX_UPLOAD_BYTES=52428800
+MAX_IMAGES=8
+HTTP_TIMEOUT_SECONDS=300
+CONNECT_TIMEOUT_SECONDS=15
+ALLOWED_ORIGINS=
+GOOGLE_MODEL_NANO_BANANA=gemini-2.5-flash-image
+OPENROUTER_MODEL_NANO_BANANA=google/gemini-2.5-flash-image