feat: unify on single repo-root compose file for dev and prod

.env.example

@@ -1,16 +1,10 @@
 # Copy to `.env` and fill in values. `.env` is gitignored.
+# It holds only ubiquitous variables (identical value in dev and prod).
+# Dev-only variables travel via devcontainer.json `containerEnv`
+# (e.g. APP_ENV=dev); paths resolve from the repo root in code.
 FORGEJO_TOKEN=
 
-# --- app database (SQLite, default if unset) ---
-# Override only if you want a different engine/URL.
-# DATABASE_URL=sqlite:///backend/data/kirche.db
-
-# --- canonical flow definitions ---
-# FLOWS_DIR=backend/flows
-# Auto-reconcile backend/flows into DB + LFX while the API runs.
-# FLOWS_WATCH_ENABLED=true
-
-# --- langflow (authoring, dev only) ---
+# --- langflow (authoring UI) ---
 LANGFLOW_URL=http://langflow:7860
 LANGFLOW_SUPERUSER=langflow
 LANGFLOW_SUPERUSER_PASSWORD=
@@ -24,7 +18,10 @@ LFX_URL=http://lfx:6004
 LFX_API_KEY=lfx-
 
 # --- API self-reference for flow callbacks (must be reachable FROM lfx) ---
-CALLBACK_BASE_URL=http://app:8000
+CALLBACK_BASE_URL=http://api:8000
+
+# --- frontend mode: dev server with HMR (prod .env must set `preview`) ---
+FRONTEND_MODE=dev
 
 # --- LLM (optional; tests mock the provider without it) ---
 # LLM_API_KEY=