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/modules/gebos-ingester.nix

@@ -9,10 +9,12 @@ in
     enable = lib.mkEnableOption "Gebos MQTT → Postgres ingester";
     mqttBroker = lib.mkOption {
       type = lib.types.str;
+      default = "tcp://127.0.0.1:1883";
       example = "tcp://127.0.0.1:1883";
     };
     postgresUrl = lib.mkOption {
       type = lib.types.str;
+      default = "postgres://gebos_ingest@127.0.0.1:5432/postgres?sslmode=require";
       description = "DSN without password — password comes from EnvironmentFile.";
     };
   };
@@ -32,7 +34,8 @@ in
       serviceConfig = {
         ExecStart = "${ingester}/bin/gebos-ingester";
         DynamicUser = true;
-        EnvironmentFile = "/etc/gebos/secrets.env";  # GEBOS_POSTGRES_PASSWORD
+        # GEBOS_POSTGRES_PASSWORD. Rendered by sops-nix — see gebos-secrets module.
+        EnvironmentFile = config.services.gebos.secrets.envFile;
         Restart = "on-failure";
         RestartSec = "5s";
       };