fix(backup): make a restored device match the one that was backed up

Found by wiping a working device and reinstalling from scratch. Every
problem here is invisible until you actually do that, which is why a
green test suite and eleven hours of uptime had not surfaced any of them.

**Four enabled plugins vanished on restore.** Weather, stocks, music and
leaderboard have a registry `id` that differs from the `id` in their own
manifest: the registry calls them `weather`, everything else calls them
`ledmatrix-weather`. Installation already prefers the manifest id for the
directory name and warns when the two disagree, so on disk, in
config.json and in a backup they are `ledmatrix-weather` -- but nothing
resolved that in reverse. Restore asked the store for `ledmatrix-weather`
and got "Plugin not found in registry", four times, and the device came
back missing four plugins the user had enabled.

Registry lookup now falls back to matching `plugin_path`, which already
records `plugins/ledmatrix-weather`. Renaming the published ids would
have orphaned `plugin_state.json` entries keyed on the old ones. Exact id
still wins, so a path that collides with another entry's id cannot
shadow it. Against the live registry and a real 28-plugin install this
takes unresolvable directories from five to one -- the one being
starlark-apps, which is genuinely not in the registry.

**Secrets could not be restored at all.** A fresh install left
config_secrets.json group-readable but not group-writable, and the web
interface -- which is what performs a restore -- does not necessarily run
as the owner. Every other file in the backup restored; secrets failed
with EACCES. Now group-writable, so the account running the web UI can
put them back.

**A partial restore reported "Restore had errors" and nothing else.**
That is the same message whether the whole thing failed or it quietly
dropped your API keys. It now names what was restored, what failed, and
which plugins were not reinstalled.

**ytm_auth.json was never in the backup.** It sits in config/ beside the
three files that are, and is pure device-local auth: losing it silently
signs the user out of YouTube Music. Backed up and restored with the
wifi config, which it resembles.

**Backups were written inside the directory a reinstall deletes.**
config/backups/exports is destroyed by the reinstall the user was told to
make it before. Exports now go beside the install, falling back to the
old path when that is not writable.

**The installer reboots without asking in non-interactive mode**, which
the README did not mention -- easy to hit when piping the install, and
alarming when a device you are installing onto disappears. Documented,
with --no-reboot-prompt. Its log also claimed root:ledmatrix while
printing a hardcoded group name rather than the one it used.

Tests: registry resolution gets its own suite, including the collision
case and third-party entries with an empty plugin_path. The existing
round-trip test passed throughout this because its fixture plugin has a
directory name equal to its id -- the one shape that cannot fail -- so it
now carries ytm_auth too.

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

web_interface/blueprints/api_v3.py

@@ -7848,7 +7848,31 @@ def clear_old_errors():
 # Backup / Restore
 # ---------------------------------------------------------------------------
 
-_BACKUP_EXPORT_DIR = PROJECT_ROOT / "config" / "backups" / "exports"
+def _resolve_backup_export_dir() -> Path:
+    """Where exported backups live: beside the install, not inside it.
+
+    They used to be written to ``<project>/config/backups/exports``. That is
+    inside the directory a reinstall deletes, so the documented recovery path
+    -- export a backup, then reinstall -- destroyed the backup it had just
+    told the user to make. Anyone who downloaded the ZIP was fine; anyone
+    relying on the on-device copy was not.
+
+    Falls back to the old location when the parent directory is not writable,
+    so an unusual layout degrades to previous behaviour instead of failing to
+    export at all.
+    """
+    preferred = PROJECT_ROOT.parent / "ledmatrix-backups"
+    try:
+        preferred.mkdir(parents=True, exist_ok=True)
+        probe = preferred / ".writetest"
+        probe.write_text("", encoding="utf-8")
+        probe.unlink()
+        return preferred
+    except OSError:
+        return PROJECT_ROOT / "config" / "backups" / "exports"
+
+
+_BACKUP_EXPORT_DIR = _resolve_backup_export_dir()
 
 
 def _safe_backup_path(filename: str) -> Path:
@@ -8002,7 +8026,22 @@ def backup_restore():
 
         data = result.to_dict()
         if not result.success:
-            return jsonify({'status': 'error', 'message': 'Restore had errors', 'data': data}), 500
+            # Name what failed, and what nonetheless landed. A restore is
+            # partial far more often than it is total -- a fresh install can
+            # leave config_secrets.json unwritable by the web service, so
+            # config restores and secrets do not. "Restore had errors" alone
+            # left the user unable to tell a wholly failed restore from one
+            # that quietly dropped their API keys.
+            failed_plugins = [p.get('plugin_id') for p in (result.plugins_failed or [])]
+            parts = []
+            if result.restored:
+                parts.append(f"restored: {', '.join(result.restored)}")
+            if result.errors:
+                parts.append(f"failed: {'; '.join(result.errors)}")
+            if failed_plugins:
+                parts.append(f"plugins not reinstalled: {', '.join(failed_plugins)}")
+            message = 'Restore incomplete — ' + ('. '.join(parts) if parts else 'see logs')
+            return jsonify({'status': 'error', 'message': message, 'data': data}), 500
         return jsonify({'status': 'success', 'data': data})
     except Exception as e:
         logger.error("backup_restore failed: %s", e, exc_info=True)