fix(update): warn when the cumulative changelog could not be fully loaded (v1.38.3)

[iscarelli]

Why

/admin/update shows the notes of every version between the installed one and the latest release (since v1.38.0). When the raw CHANGELOG.md fetch or the slicing fails, cumulative_release_notes() falls back to the last release's body — silently. A truncated history was indistinguishable from a complete one, so someone several versions behind would believe nothing else had changed.

What changed

• cumulative_release_notes() now returns (notes, complete) — complete is True only when the CHANGELOG was really sliced, False on every fallback path (no tag, fetch raised, slice matched nothing). The fail-open behaviour is unchanged: it still never raises and never blanks the card.
• New notes_incomplete_warning(complete, current, latest) decides whether the warning is worth showing. It stays quiet for a single patch bump directly above the installed version, where the last release's notes already are the whole story.
• /admin/update renders a discreet text-muted warning inside the existing collapse, with a link to the GitHub releases page.
• New strings translated in _EN and _ES.

Verify

.\run-tests.ps1 → 179 passed. New coverage: the fallback path still returns the last-good notes and reports complete=False; the happy path reports True with more than one version section; three unit tests for the warning rule; two rendering tests on /admin/update (warning present when the fallback is forced, absent when the slicing works).

Note for future test authors

routes/admin.py does from app import current_version — a plain-name import bound at import time. monkeypatch.setattr(app_module, "current_version", ...) therefore does not change what the route sees. The new rendering tests use the real VERSION value instead of faking it.

🤖 Generated with Claude Code