fix(web-ui): dedup registry fetches, surface reconciliation warnings, add check-update endpoint

Story 1 — src/plugin_system/store_manager.py:
Add threading.Lock (_registry_fetch_lock) to fetch_registry(). The outer cache
check remains the hot path (no lock). When the cache is cold, only one thread
hits the network; concurrent callers block on the lock then get the result from
the warm cache (double-checked locking). Eliminates duplicate GitHub requests
on every page load when the 15-minute cache expires.

Story 2 — web_interface/app.py + api_v3.py + overview.html:
_run_startup_reconciliation() now writes /tmp/ledmatrix_reconciliation.json
(atomic tempfile+replace, mirrors hw_status pattern) so the result survives
the background thread. New GET /api/v3/plugins/reconciliation-status reads
that file. Overview page gains a dismissible yellow banner that shows stale
plugin_id values (e.g. sync, github, youtube) and tells the user to remove
them or reinstall from the Plugin Store. Banner is suppressed for the session
after dismiss using sessionStorage keyed on the plugin_id list.

Story 3 — web_interface/blueprints/api_v3.py:
Add GET /api/v3/system/check-update. Does git fetch origin main then compares
local HEAD vs origin/main to compute update_available, remote_sha, and
commits_behind. Result is cached for 5 minutes so it doesn't run git on every
page load. Falls back to {update_available: false} on any error. Eliminates
the 404 logged on every page load.

Story 4 (Pi 5 rgbmatrix rebuild) was already fixed in PR #341.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/app.py

@@ -716,6 +716,33 @@ def _run_startup_reconciliation() -> None:
                 "manual 'Reconcile' action to resolve.",
                 len(result.inconsistencies_manual),
             )
+
+        # Write status file so the web UI can surface unresolved issues as a
+        # banner without the user having to read journalctl. Mirrors the
+        # hw_status pattern (/tmp/led_matrix_hw_status.json).
+        import json as _json, tempfile as _tempfile, os as _os
+        _recon_status = {
+            "done": True,
+            "successful": result.reconciliation_successful,
+            "fixed_count": len(result.inconsistencies_fixed),
+            "unresolved": [
+                {
+                    "plugin_id": inc.plugin_id,
+                    "type": inc.inconsistency_type.value,
+                    "description": inc.description,
+                }
+                for inc in result.inconsistencies_manual
+            ],
+        }
+        _recon_path = "/tmp/ledmatrix_reconciliation.json"
+        try:
+            if not _os.path.islink(_recon_path):
+                _fd, _tmp = _tempfile.mkstemp(dir="/tmp", prefix=".led_recon_")
+                with _os.fdopen(_fd, "w") as _f:
+                    _json.dump(_recon_status, _f)
+                _os.replace(_tmp, _recon_path)
+        except Exception as _e:
+            _logger.warning("[Reconciliation] Could not write status file: %s", _e)
     except Exception as e:
         _logger.error("[Reconciliation] Error: %s", e, exc_info=True)
     finally: