fix(deploy): booth.sh works in the flat /opt layout; .env TAG=dev default

The booth deploys the compose files FLAT (e.g. /opt/parking_systems/) with
booth.sh next to them, but the script assumed it lived in <repo>/scripts/ and
blindly did `cd ..` — so REPO_DIR resolved to the parent, where there are no
compose files, and every subcommand operated on the wrong dir. `usage()` then
sed-read a relative $0 that no longer existed after the cd ("can't read
booth.sh"). Discover the compose files instead: check the script's own dir,
then ../, then $PWD, and cd to whichever has docker-compose.yml. usage() reads
an absolute $SELF so it survives the cd.

Also: .env.example defaulted TAG=main, but the registry only has dev-* tags
(no main build yet), so `compose pull` 404s. Default to TAG=dev and document
the moving-vs-immutable (dev / dev-<sha>) tag scheme.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

.env.example

@@ -5,9 +5,13 @@
 # --- image source (prod pulls from the house Gitea registry) ------------------
 # The registry namespace; combined with the image name + TAG below.
 REGISTRY=git.infra.msai.al/mca/parking_solution
-# Moving branch tag to deploy: `main` for production, `dev` for staging. A push to
-# that branch republishes this tag, so `booth.sh update` pulls the latest of it.
-TAG=main
+# Image tag to deploy. CI publishes TWO tags per build: a MOVING branch tag
+# (`dev`, and `main` once that branch is built) republished on every push, and an
+# IMMUTABLE per-commit `dev-<sha>` (e.g. dev-830993b). Use the moving tag for a
+# self-updating booth (`booth.sh update` pulls the latest); pin the `<branch>-<sha>`
+# form for a reproducible, deterministic deploy. NOTE: `main` images only exist once
+# something is built on main — until then deploy from `dev`.
+TAG=dev
 
 # --- secrets (NO safe defaults — the server refuses to boot without a real one) -
 # JWT signing secret. Generate yourself, never share it:  openssl rand -hex 32