Tag non-production tabs, and drop the FOODSTER_ prefix

With dev and prod open side by side in the same browser, the tabs were
indistinguishable. ENV is written into the title of every page unless it says
prod, so "dev · Foodster" picks itself out. The value is used verbatim, so
ENV=staging labels itself too, and prod and production both count as unmarked
so a stray capital cannot tag the real instance.

Environment variables lose their prefix: PASSWORD, DB, ENV, ADDR, HOST, REPO,
TAG. The container namespaces them already, and this matches how the other
services here are configured.

PUID/PGID are the exception rather than UID/GID. UID is read-only in bash, so
a value set in .env would be silently replaced by the invoking shell's own and
compose's user: would ignore what was asked for.

Breaking for a running instance: the deployed .env has to be rewritten in the
same deploy, or the app will refuse to start on an unset PASSWORD.

.env.example

@@ -1,22 +1,29 @@
 # Copy to .env and fill in. .env is gitignored — the real registry hostname
 # must not end up in the repository.
 
-# Image coordinates. FOODSTER_REPO carries no tag.
-FOODSTER_REPO=registry.example.com/you/foodster
-FOODSTER_TAG=latest
+# Image coordinates. REPO carries no tag.
+REPO=registry.example.com/you/foodster
+TAG=latest
 
 # Shared household password. The app will not start without it.
-FOODSTER_PASSWORD=changeme
+PASSWORD=changeme
 
 # Hostname Traefik routes to. Kept here rather than in compose.yaml so no
 # infrastructure detail is committed.
-FOODSTER_HOST=foodster.example.com
+HOST=foodster.example.com
+
+# Anything other than prod is written into the browser tab title, so a dev
+# instance open beside the real one can be told apart.
+ENV=prod
 
 # The database lives in ./data, bind-mounted into the container. These must
 # match whoever owns that directory on the host, or the container cannot
 # write to it. `id -u` and `id -g` will tell you.
-FOODSTER_UID=1000
-FOODSTER_GID=1000
+#
+# Named PUID/PGID because UID is read-only in bash and a plain UID here would
+# be quietly replaced by the invoking shell's own.
+PUID=1000
+PGID=1000
 
 # Used for every calendar-day calculation. Set it in development too: under
 # UTC the date rolls over three hours late, which is exactly when dinner