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

backend/fluksio/flow/store.py

@@ -97,6 +97,13 @@ class FlowStore:
     #: One warning per process when there is no git to commit with.
     _warned_no_git = False
 
+    #: Commits between housekeeping runs. Auto-gc is off (see `_git`), so
+    #: nothing packs unless it is asked for; a commit counter is the trigger
+    #: because the process that needs it never restarts — a seeding session
+    #: commits thousands of times inside one long-lived container, which is how
+    #: a store reached 1898 commits, 5863 loose objects and no pack at all.
+    _GC_EVERY = 500
+
     def __init__(self, root: Path) -> None:
         self.root = root
         self.root.mkdir(parents=True, exist_ok=True)
@@ -120,7 +127,13 @@ class FlowStore:
     def _git(self, *args: str) -> subprocess.CompletedProcess[str]:
         try:
             return subprocess.run(
-                ["git", "-C", str(self.root), *args],
+                # `gc.auto=0` on every invocation, because `git commit` otherwise
+                # forks a background `gc --auto` and returns without waiting for
+                # it. `subprocess.run` reaps the commit, not its grandchild, so
+                # the daemon is reparented onto PID 1 — the FastAPI process,
+                # which reaps nothing, and the zombies accumulate one per save.
+                # Packing is asked for explicitly in `_commit` instead.
+                ["git", "-C", str(self.root), "-c", "gc.auto=0", *args],
                 capture_output=True,
                 text=True,
                 check=False,
@@ -158,6 +171,11 @@ class FlowStore:
             message,
         )
         if result.returncode == 0 or "nothing to commit" in result.stdout:
+            if self.revision % self._GC_EVERY == 0:
+                # In the foreground, so this child is waited on rather than
+                # orphaned. Plain `gc` rather than `gc --auto`, which the flag
+                # above has just disabled.
+                self._git("gc", "--quiet")
             return
         if result.stderr == "git is not installed":
             # Already said once, in `_git`. Repeating it per save is noise.