ddev bootstrap: make app/.env optional, add .env.example

Fresh clones errored out on `ddev start` because the compose file's
env_file: pointed at app/.env (gitignored). No env vars are actually
required by the code — STATE_DIR is the only one and has a default.

- Switch env_file to the path:+required:false form (Docker Compose
  2.24+; bundled with DDEV) so missing app/.env is non-fatal.
- Commit app/.env.example as documentation for contributors who do
  need overrides.

app/.env.example

@@ -0,0 +1,12 @@
+# Local-dev environment variables for the emdash dev container.
+# Copy this file to `app/.env` and edit if you need to override defaults.
+# Loaded by .ddev/docker-compose.emdash.yaml (env_file, required: false) —
+# everything here is optional; the dev server boots fine with no overrides.
+
+# Override where emdash stores data.db and uploads/ on disk.
+# Defaults to the working directory (which is /app inside the container).
+# STATE_DIR=/app/state
+
+# Astro / emdash:
+# HOST, PORT, NODE_ENV, EMDASH_ALLOWED_ORIGINS, EMDASH_SITE_URL are set in
+# the compose `environment:` block — set them here only to override.