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/default.nix

@@ -1,18 +1,22 @@
 { inputs }:
 
 let
+  modules = import ../modules;
+
   mkHost = name: extraModules:
     inputs.nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       specialArgs = { inherit inputs; };
       modules = [
         ./common.nix
-        (import ../modules).gebos-postgres
-        (import ../modules).gebos-supabase
-        (import ../modules).gebos-caddy
-        (import ../modules).gebos-hivemq
-        (import ../modules).gebos-ingester
-        (import ../modules).gebos-frontend
+        inputs.sops-nix.nixosModules.sops
+        modules.gebos-secrets
+        modules.gebos-postgres
+        modules.gebos-supabase
+        modules.gebos-caddy
+        modules.gebos-hivemq
+        modules.gebos-ingester
+        modules.gebos-frontend
         ./${name}.nix
       ] ++ extraModules;
     };