Add public API key signup

.env.example

@@ -3,6 +3,11 @@ PORT=8787
 # When DATABASE_URL is set the API runs in async (queue) mode; otherwise it runs
 # clones inline in-memory (handy for a quick local single-page demo).
 # CLONE_TTL_MS=1800000        # in-memory mode: how long results are retained
+# API_KEYS=                  # comma-separated keys that can call /v1/clones and /mcp
+# RATE_LIMIT_PER_MINUTE=60   # service-wide cap for authenticated clone/MCP calls
+# SIGNUP_ENABLED=false       # DB mode only: expose POST /v1/signup to mint API keys
+# SIGNUP_RATE_LIMIT_PER_HOUR=3
+# DEFAULT_SIGNUP_KEY_RATE_LIMIT=30  # stored on minted keys; current limiter is service-wide
 
 # ---- Database + queue (Postgres; pg-boss uses the same DB) ----
 DATABASE_URL=postgresql://postgres:postgres@localhost:5432/ditto_site