fix(web): keep exception messages out of API responses (py/stack-trace-exposure)

CodeQL had ~40 open py/stack-trace-exposure alerts on main. Almost all
flowed through describe_exception(), which returned "TypeName: message"
(redacted, capped); the rest through _run_systemctl_command's str(err),
WiFiManager's `return False, str(e)`, unit_refresh's f-strings and two
str(e)/f"{err}" messages in api_v3/__init__.py.

describe_exception() now returns a reason code -- the type, plus the
errno symbol for an OSError ("OSError:EIO", "PermissionError:EACCES") --
and logs the redacted message itself. That keeps what #538 wanted (a
failing disk still says EIO in the response) without quoting paths,
URLs or library internals, and fixes every call site at once; the
test_no_api_v3_handler_discards_its_exception policy still holds.

Service results: _get_display_service_status returns active/returncode
only, and the on-demand start/stop `service` result keeps
returncode/active/started/status but drops systemctl stdout/stderr
(logged on failure). Nothing in web_interface/static, the templates or
the MQTT bridge reads those fields. The Starlark SIGKILL-restart error
no longer returns systemctl stderr as `details`.

WiFi, unit-refresh, config-save and plugin-removal failures now say
what failed with the reason code and point at the log. display.py is
untouched (draft #773 edits it).

Tests: test_api_v3_no_exception_text.py drives one route per affected
file with a marker in the exception message and asserts it never
reaches the body; all 13 fail on origin/main, and targeted mutations
(drop the service filter, put stderr back, str(e) in WiFiManager,
{e} in unit_refresh, {install_err} in system.py, message back in
describe_exception) each fail at least one. Tests that asserted the old
message-in-details contract now assert the reason code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_api_v3_registry_endpoints.py

@@ -95,9 +95,10 @@ class TestRefreshPluginStore:
             RuntimeError("failed at /home/user/LEDMatrix/src/secret.py line 42"))
         body = api_v3_client.post(self.URL, json={}).get_json()
         assert "Traceback" not in str(body)
-        # `details` is describe_exception output: one line, type-named,
-        # credential-redacted. It may quote the message, but never a stack.
-        assert body["details"].startswith("RuntimeError:")
+        # `details` is describe_exception output: the type, never the
+        # message or a stack.
+        assert body["details"] == "RuntimeError"
+        assert "secret.py" not in str(body)
         assert "\n" not in body["details"]