Add sops-nix secrets + dev defaults so local stack runs with zero setup

- sops-nix flake input, age + ssh-to-age in devShell, .sops.yaml with
  per-host recipients (real pubkeys still TODO until lars bootstraps).
- nix/modules/gebos-secrets.nix centralises per-host secret subsets and
  renders /run/secrets/gebos-env via sops.templates. Service modules now
  read EnvironmentFile= from services.gebos.secrets.envFile instead of
  the placeholder /etc/gebos/secrets.env.
- Falls back to a writeText env file when nix/secrets/secrets.yaml is
  absent, so `nix flake check` and first-boot eval work pre-bootstrap.
- nix/secrets/README.md walks through age key + sops bootstrap.
- Dev defaults: ingester binary falls back to local tcp/postgres URLs,
  process-compose adds mosquitto and wires GEBOS_MQTT_BROKER /
  GEBOS_POSTGRES_URL / GEBOS_POSTGRES_PASSWORD + VITE_SUPABASE_URL /
  VITE_SUPABASE_ANON_KEY, frontend defaults to 127.0.0.1:8000 in DEV.

Refs #23 (comment #110).

frontend/src/supabase.ts

@@ -1,7 +1,20 @@
 import { createClient } from "@supabase/supabase-js";
 
 // Single subdomain for everything Supabase — Kong fans out to /auth/v1, /rest/v1, /rpc.
-const SUPABASE_URL = import.meta.env.VITE_SUPABASE_URL ?? "https://api.ge-bos.de";
-const SUPABASE_ANON_KEY = import.meta.env.VITE_SUPABASE_ANON_KEY ?? "";
+//
+// Defaults match the dev stack in nix/dev/process-compose.nix so `npm run dev`
+// works without an .env file. Production bundles are built with VITE_SUPABASE_URL
+// and VITE_SUPABASE_ANON_KEY set at `nix build .#frontend` time.
+const DEV_SUPABASE_URL = "http://127.0.0.1:8000";
+const DEV_SUPABASE_ANON_KEY =
+  "dev-anon-key-placeholder-regenerate-with-supabase-self-host-script";
+
+const SUPABASE_URL =
+  import.meta.env.VITE_SUPABASE_URL ??
+  (import.meta.env.DEV ? DEV_SUPABASE_URL : "https://api.ge-bos.de");
+
+const SUPABASE_ANON_KEY =
+  import.meta.env.VITE_SUPABASE_ANON_KEY ??
+  (import.meta.env.DEV ? DEV_SUPABASE_ANON_KEY : "");
 
 export const supabase = createClient(SUPABASE_URL, SUPABASE_ANON_KEY);