fix(store): serialize concurrent installs, and make the lock reentrant

Second bug found while validating the previous commit on hardware.

install_plugin's new set-aside/restore had no lock. The web UI runs Flask
threaded, so a double-clicked Install button gives two threads the same
plugin_id; interleaved, one thread's restore deletes the other's freshly
installed copy. _reinstall_with_rollback already guards exactly this with a
per-plugin lock, and install_plugin needs the same one.

Taking that lock naively deadlocks. _reinstall_with_rollback holds it across
its call to install_plugin, and threading.Lock is not reentrant -- so the
request thread hangs forever on the standard monorepo update path
(update_plugin -> _reinstall_with_rollback -> install_plugin), which is to say
on every plugin update. Verified by reverting to a plain Lock: the regression
test times out after 10s instead of passing.

The per-plugin locks are now RLocks, and install_plugin holds one for its
whole set-aside/install/restore sequence.

Verified on devpi (Pi, Python 3.13.5, real registry and network):
- update_plugin on an up-to-date plugin: True in 5.4s
- update_plugin forced through the full reinstall-with-rollback path:
  True in 13.1s, correct version restored, old copy replaced, no backup
  directories left behind
- install -> reinstall-over-existing -> failed-reinstall-restores: all pass
  against real downloads
- 22 plugins load, no tracebacks, web API and UI 200, steady-state journal
  50 lines/min

791 core unit tests pass, including 2 new concurrency tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

test/test_install_preserves_existing.py

@@ -150,3 +150,73 @@ class TestUpdatePathStillWorks:
         assert observed["dirs"] == ["hockey-scoreboard.standalone-backup-migrating"]
         # And the user still has their plugin.
         assert (plugins_dir / "hockey-scoreboard" / "marker.txt").read_text() == "the-original"
+
+
+class TestConcurrency:
+    """The web UI runs Flask threaded, so a double-clicked Install button puts
+    two threads on the same plugin_id. `_reinstall_with_rollback` already
+    guarded against this; the install wrapper has to as well, or one thread's
+    restore deletes the other's freshly installed copy."""
+
+    def test_rollback_calling_install_does_not_deadlock(self, store, monkeypatch):
+        """The rollback path holds the per-plugin lock across its call to
+        install_plugin. A non-reentrant lock would hang the request thread
+        forever — this test would time out rather than fail."""
+        import threading
+
+        mgr, plugins_dir = store
+        path = _existing_install(plugins_dir, "hockey-scoreboard", "the-original")
+        monkeypatch.setattr(
+            mgr, "_install_plugin_impl",
+            lambda pid, branch=None: bool(_existing_install(plugins_dir, pid, "new")))
+
+        done = threading.Event()
+        result = {}
+
+        def run():
+            result["ok"] = mgr._reinstall_with_rollback("hockey-scoreboard", path)
+            done.set()
+
+        t = threading.Thread(target=run, daemon=True)
+        t.start()
+        assert done.wait(timeout=10), (
+            "install_plugin deadlocked when called from _reinstall_with_rollback "
+            "— the per-plugin lock must be reentrant"
+        )
+        assert result["ok"] is True
+
+    def test_concurrent_installs_serialize(self, store, monkeypatch):
+        """Two threads installing the same plugin must not interleave their
+        set-aside/restore, and the survivor must be a complete install."""
+        import threading
+
+        mgr, plugins_dir = store
+        _existing_install(plugins_dir, "hockey-scoreboard", "the-original")
+
+        in_flight = []
+        overlap = []
+
+        def slow_impl(plugin_id, branch=None):
+            in_flight.append(1)
+            if len(in_flight) > 1:
+                overlap.append(1)
+            threading.Event().wait(0.05)
+            _existing_install(plugins_dir, plugin_id, "installed")
+            in_flight.pop()
+            return True
+
+        monkeypatch.setattr(mgr, "_install_plugin_impl", slow_impl)
+
+        threads = [threading.Thread(target=mgr.install_plugin,
+                                    args=("hockey-scoreboard",), daemon=True)
+                   for _ in range(2)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join(timeout=10)
+            assert not t.is_alive(), "concurrent install hung"
+
+        assert not overlap, "two installs of the same plugin ran concurrently"
+        assert (plugins_dir / "hockey-scoreboard" / "marker.txt").exists()
+        leftovers = [p.name for p in plugins_dir.iterdir() if "backup" in p.name]
+        assert not leftovers, f"backup left behind: {leftovers}"