feat: compose stack in devcontainer (db, langflow, lfx)

- docker-compose.yml with app/db/langflow/lfx services and healthchecks
- Dockerfile.lfx (stateless flow runtime on :6004)
- init-db.sql (langflow + test databases)
- devcontainer.json switched to compose mode with runServices
- setup.sh waits for compose services and requires .env

.devcontainer/setup.sh

@@ -13,6 +13,38 @@ echo "--- Installing Node dependencies ---"
 cd /workspaces/kirche-einfach/frontend
 npm install
 
+# Compose services (db, langflow, lfx) must be up; fail loudly otherwise.
+echo "--- Waiting for compose services ---"
+if [ ! -f /workspaces/kirche-einfach/.env ]; then
+  echo "ERROR: /workspaces/kirche-einfach/.env is missing. Copy .env.example to .env and fill in secrets." >&2
+  exit 1
+fi
+python3 - <<'EOF'
+import socket
+import sys
+import time
+
+targets = [("db", 5432), ("langflow", 7860), ("lfx", 6004)]
+deadline = time.time() + 180
+pending = dict(targets)
+while pending and time.time() < deadline:
+    for host, port in list(pending):
+        s = socket.socket()
+        s.settimeout(2)
+        try:
+            s.connect((host, port))
+            print(f"OK {host}:{port}", flush=True)
+            del pending[(host, port)]
+        except OSError:
+            pass
+        finally:
+            s.close()
+    if pending:
+        time.sleep(2)
+if pending:
+    sys.exit(f"ERROR: compose services not ready: {sorted(pending)}")
+EOF
+
 echo ""
 echo "=== Setup complete ==="
 echo "Forgejo username: $FORGEJO_USERNAME"