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).

nix/hosts/mqtt-ingest.nix

@@ -3,6 +3,8 @@
 {
   networking.hostName = "mqtt-ingest";
 
+  services.gebos.secrets.ingester = true;
+
   services.gebos.hivemq = {
     enable = true;
     # Persistent sessions on disk — single-node, devices reconnect on restart.
@@ -12,9 +14,9 @@
 
   services.gebos.ingester = {
     enable = true;
-    mqttBroker = "tcp://127.0.0.1:1883";   # local broker
+    # mqttBroker defaults to tcp://127.0.0.1:1883 (the local HiveMQ).
     postgresUrl = "postgres://gebos_ingest@db-host.gebos.internal:5432/postgres?sslmode=require";
-    # Password sourced from /etc/gebos/secrets.env via EnvironmentFile.
+    # GEBOS_POSTGRES_PASSWORD sourced from /run/secrets/gebos-env (sops-nix).
   };
 
   # Caddy fronts TLS for `ingest.ge-bos.de` and forwards to HiveMQ.