docs: document pluggable storage adapter env vars

.env.example

@@ -1,25 +1,31 @@
-# StableRoute backend — environment variable template.
-# Copy this file to `.env` and adjust as needed. `.env` is git-ignored.
-
-# Port the HTTP server listens on. Defaults to 3001 when unset.
-PORT=3001
-
-# Grace period (in milliseconds) before the shutdown handler forces process.exit(1)
-# when server.close() is still draining connections. Defaults to 10000 (10 s).
-# Must be a positive integer; any other value is silently ignored and the default
-# of 10 000 ms is used instead.
-SHUTDOWN_GRACE_MS=10000
-
-# Runtime mode. Use "development" locally and "production" in deployment.
-# Setting it to "test" disables the rate limiter and per-request logging
-# (used by the Jest suite); do not use "test" outside of tests.
-NODE_ENV=development
-
-# Commit SHA injected by the CI/CD pipeline and surfaced by GET /api/v1/version.
-# Leave unset (or blank) for local development; the endpoint falls back to "unknown".
-# GIT_COMMIT=
-
-# Build timestamp injected by the CI/CD pipeline and surfaced by GET /api/v1/version.
-# ISO-8601 format recommended (e.g. 2026-01-01T00:00:00Z).
-# Leave unset (or blank) for local development; the endpoint falls back to "unknown".
-# BUILD_TIME=
+# StableRoute backend — environment variable template.
+# Copy this file to `.env` and adjust as needed. `.env` is git-ignored.
+
+# Port the HTTP server listens on. Defaults to 3001 when unset.
+PORT=3001
+
+# Grace period (in milliseconds) before the shutdown handler forces process.exit(1)
+# when server.close() is still draining connections. Defaults to 10000 (10 s).
+# Must be a positive integer; any other value is silently ignored and the default
+# of 10 000 ms is used instead.
+SHUTDOWN_GRACE_MS=10000
+
+# Runtime mode. Use "development" locally and "production" in deployment.
+# Setting it to "test" disables the rate limiter and per-request logging
+# (used by the Jest suite); do not use "test" outside of tests.
+NODE_ENV=development
+
+# Commit SHA injected by the CI/CD pipeline and surfaced by GET /api/v1/version.
+# Leave unset (or blank) for local development; the endpoint falls back to "unknown".
+# GIT_COMMIT=
+
+# Build timestamp injected by the CI/CD pipeline and surfaced by GET /api/v1/version.
+# ISO-8601 format recommended (e.g. 2026-01-01T00:00:00Z).
+# Leave unset (or blank) for local development; the endpoint falls back to "unknown".
+# BUILD_TIME=
+
+# Persistent storage backend (GrantFox #70).
+# memory — default; state is lost on restart.
+# json-file — persist to STORAGE_FILE (survives restarts).
+STORAGE_BACKEND=memory
+# STORAGE_FILE=./stableroute-data.json