v0.5.0: TrueNAS alert when an update is available

Raises a real alert in the TrueNAS UI bell -- not a log line nobody reads. On by
default, checked once a day. install.sh --no-update-alerts turns it off.

It does not nag
---------------
A release whose CHANGELOG contains only a "### Docs" section changed no code and
raises nothing. Anything else raises INFO; a "### Security" section raises WARNING.

The CHANGELOG's own section headings are the signal, and a security fix anywhere in
the range escalates the whole span -- a docs-only release sitting on top of a
security fix still reports as security rather than hiding it.

Why an AlertSource and not midclt
----------------------------------
TrueNAS cannot raise an alert from the CLI. midclt exposes only alert.dismiss,
alert.list, alert.list_categories, alert.list_policies and alert.restore -- alert
CREATION is internal to middlewared, and none of its ~60 one-shot classes is
generic enough to reuse. Registering an AlertSource is the only way.

It is also the least invasive thing this patch does. The providers and nested
modules both APPEND CODE TO STOCK middleware files; the alert source ADDS ONE FILE
and modifies none. It is the native mechanism -- the same one every built-in
TrueNAS alert uses -- and TrueNAS polls it itself, so there is no cron job and no
systemd timer.

- Fail-safe: every error path returns None; it cannot take middlewared down.
- Read-only: `git ls-remote` plus an HTTPS fetch of the CHANGELOG. It never writes
  to .git, so it cannot leave root-owned objects behind the way a `git fetch` from
  middlewared (running as root) would.
- Removed by uninstall.sh (mw_patch.revert_all).
- It only tells you; it never updates anything.

Verified against the real repo and remote, with middlewared stubbed:
  on v0.4.1, only a README-only v0.4.2 available -> NO ALERT
  on v0.4.0, v0.4.1 fixed real bugs             -> INFO
  on v0.3.2, v0.3.3 was the password fix        -> SECURITY / WARNING

139 tests, ruff and shellcheck -S style clean.

uninstall.sh

@@ -3,7 +3,7 @@
 
 set -euo pipefail
 
-VERSION="0.4.2"
+VERSION="0.5.0"
 
 PATCH_DIR="$(cd "$(dirname "$0")" && pwd)"
 _HOOK_COMMENT='TrueCloud provider patch (S3/B2)'