fix(deploy): let app own secret key — remove STEWARD_SECRET_KEY from deploy compose

The app auto-generates a key on first boot and persists it to /data/secret.key,
which the app_data volume keeps across image updates. No reason to surface this
as a user-facing env var in the deploy compose. Removed from .env.example too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

compose.deploy.yml

@@ -23,10 +23,9 @@ services:
       - app_data:/data
     environment:
       - STEWARD_DATABASE_URL=postgresql+asyncpg://steward:${POSTGRES_PASSWORD:-steward}@db/steward
-      # Optional: if unset, the app generates a key and persists it to
-      # /data/secret.key, so sessions survive restarts via the app_data volume.
-      # Set STEWARD_SECRET_KEY in .env to pin it explicitly across volume resets.
-      - STEWARD_SECRET_KEY=${STEWARD_SECRET_KEY:-}
+      # STEWARD_SECRET_KEY is intentionally absent — the app generates a random
+      # key on first boot and persists it to /data/secret.key, which the app_data
+      # volume keeps across image updates.
     depends_on:
       db:
         condition: service_healthy