Stop the flow store leaking git zombies, honour VITE_API_URL from .env

`FlowStore._git` now passes `-c gc.auto=0`, so `git commit` no longer forks a
background `gc --auto` that reparents onto PID 1 and stays there unreaped. With
auto-gc off nothing packs on its own, so `_commit` runs a foreground `git gc`
every 500 commits — the trigger a long-lived seeding session actually reaches.

`docker/compose.yml` takes the frontend's `VITE_API_URL` build arg from the
environment, keeping `https://api.${DOMAIN}` only as the fallback. `setup.sh`
already derives the scheme from `ENVIRONMENT`, so an `up --build` that does not
layer `compose.local.yml` stops shipping a bundle that calls `https://api.localhost`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01StpRc2C6au1WJ1EUU7fsfu

docs/reference/configuration.md

@@ -65,6 +65,11 @@ The dashboard is a static bundle, so this one is a **build** argument of the
 |---|---|---|
 | `VITE_API_URL` | `frontend` at build time | the address the interface calls |
 
+The compose stack takes it from `.env`, falling back to `https://api.${DOMAIN}`.
+`scripts/setup.sh` writes it there with the scheme `ENVIRONMENT` implies, so a
+local build calls `http://` and does not fail a certificate check nothing is
+there to satisfy.
+
 Empty is the useful value: the interface then addresses the API relative to
 whichever origin served the page, so one image answers on a hostname, on a
 `http://<host-ip>:<port>`, and through an ssh tunnel alike — and no origin has