fix(update): target docker-compose.production.yml in dashboard update steps

Production installs use docker-compose.production.yml (the README's documented
path, pinned GHCR images, no dev services), but the dashboard's update
instructions emitted bare `docker compose pull` / `up -d`. Bare `docker compose`
operates on docker-compose.yml — a different, build-based stack — so a
production user who followed the steps:
  - never pulled/recreated their real containers (stayed on the old version,
    e.g. stuck on 3.44.0 after "updating" to 3.45.2), and
  - started the dev-only mailhog service that docker-compose.yml defines
    (reported restart-looping).

The backend runs inside a container and can't stat the host's compose files, but
docker-compose.production.yml passes PICPEAK_RELEASE_CHANNEL into the backend env
and docker-compose.yml does not. detectEnvironment() now derives
isProductionCompose from it, and the Docker update steps prepend
`-f docker-compose.production.yml` when set. The non-production branch keeps the
bare commands but the warning now tells users to add `-f docker-compose.production.yml`
if they installed with it.

Also gates the mailhog service in docker-compose.yml behind a `dev` compose
profile so a plain `docker compose up -d` never starts it (opt in with
`docker compose --profile dev up -d`). Nothing depends on it (SMTP_HOST comes
from .env), so gating is safe. Verified: `docker compose config` lists mailhog
only with `--profile dev`; production compose is unchanged.

Adds unit tests for the production-vs-default command generation.

backend/src/services/environmentService.js

@@ -47,11 +47,23 @@ async function detectEnvironment() {
     type = 'standalone';
   }
 
+  // Detect a production compose install. The backend runs INSIDE a container and
+  // cannot see the host's compose files (the image only carries backend/), so we
+  // can't stat docker-compose.production.yml. Instead we key off an env var the
+  // production compose sets in the backend environment (PICPEAK_RELEASE_CHANNEL)
+  // and the default docker-compose.yml does not. When present, the update
+  // instructions must target that file explicitly — bare `docker compose`
+  // operates on docker-compose.yml, a different (build-based) stack that also
+  // starts the dev-only mailhog and leaves the real production containers on the
+  // old version.
+  const isProductionCompose = Boolean(process.env.PICPEAK_RELEASE_CHANNEL);
+
   return {
     type,
     isDocker,
     isGit,
     hasDockerCompose,
+    isProductionCompose,
     platform: process.platform,
     nodeVersion: process.version,
     appVersion
@@ -94,25 +106,36 @@ function generateUpdateInstructions(env, targetVersion) {
 
   if (env.isDocker) {
     instructions.environmentName = 'Docker';
+    // Production installs use docker-compose.production.yml (the file the README
+    // documents and the only one with pinned GHCR images + no dev-only mailhog).
+    // Bare `docker compose` targets docker-compose.yml instead, so a production
+    // user who runs it stays on the old version and gets a stray mailhog. When we
+    // detect a production compose (PICPEAK_RELEASE_CHANNEL set), point every
+    // command at that file with `-f`.
+    const composeFile = env.isProductionCompose ? '-f docker-compose.production.yml ' : '';
     instructions.steps = [
       {
         description: 'Pull latest images',
-        command: 'docker compose pull',
+        command: `docker compose ${composeFile}pull`,
         note: 'Downloads the new version images'
       },
       {
         description: 'Recreate containers with new images',
-        command: 'docker compose up -d',
+        command: `docker compose ${composeFile}up -d`,
         note: 'Restarts containers with new version'
       },
       {
         description: 'Watch logs for startup (optional)',
-        command: 'docker compose logs -f backend',
+        command: `docker compose ${composeFile}logs -f backend`,
         note: 'Press Ctrl+C to exit logs',
         optional: true
       }
     ];
-    instructions.warnings.push('Make sure you are in the directory containing your docker-compose.yml file');
+    if (env.isProductionCompose) {
+      instructions.warnings.push('Run these from the directory containing your docker-compose.production.yml file.');
+    } else {
+      instructions.warnings.push('Make sure you are in the directory containing your compose file. If you installed with docker-compose.production.yml, add `-f docker-compose.production.yml` to each command.');
+    }
   } else if (env.isGit) {
     instructions.environmentName = 'Git (Development)';
     instructions.steps = [