auth: affirmative versioned sign-up acknowledgement (EN + IT)

Replaces the passive "by signing in you agree…" paragraph on /login with
an active un-pre-ticked checkbox over three short statements that the
user actively accepts. Each acceptance is recorded against a code-level
version pointer, so a future wording revision bumps the version and
prompts every user again.

- New app/legal.py::ACKNOWLEDGEMENT_VERSION (currently 1).
- New UserAcknowledgement model + migration 0027 (user_id FK CASCADE,
  version, lang, accepted_at, composite index on user_id+version).
- auth_service: has_acknowledged_current() and record_acknowledgement()
  helpers; POST /login validates the checkbox, falls through to a 400
  with the localised error otherwise, and writes a row iff the user has
  no current-version row (so existing-already-accepted users don't
  produce duplicates).
- GET /login: language detection mirrors the landing's
  detect_public_lang(); ?lang=en|it overrides; stamps the rtm.lang
  cookie; passes the locale dict + version into the template.
- login.html: EN/IT pill, localised lede/banner/legal footer, required
  checkbox in an acknowledgement block, hidden lang+ack_version fields.
  Submit disabled until the box is ticked (UX polish; the server check
  is what carries weight).
- locales/{en,it}.yaml: new auth.ack.* section with TODO(legal) marker.
  Wording matches the brief's substance pending solicitor sign-off.
- tests/test_signup_acknowledgement.py: 10 tests (EN + IT rejection,
  one row per acceptance, displayed-lang recorded, idempotent on
  current version, version-bump writes new row, helper unit tests).

The acknowledgement strengthens the user-civil-claim vector — combined
with the liability cap and the Ltd, it makes "I was misled into thinking
this was advice" much harder to argue. It does NOT move the regulatory
perimeter, which is governed by the content discipline shipped in
47dce1a. Belt-and-braces, not a substitute.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/legal.py

@@ -0,0 +1,19 @@
+"""Legal-surface constants.
+
+Currently a one-liner: the version of the sign-up acknowledgement text
+the user actively accepts at ``/login``.
+
+Bumping this constant re-prompts every existing user on their next
+sign-in, because the new value won't match their stored
+``user_acknowledgements.version`` rows. The recorded row is what gives
+the acknowledgement its evidentiary weight; the constant is the pointer
+that says "the current canonical text is at version N". The text itself
+lives in ``app/locales/{en,it}.yaml`` under ``auth.ack``.
+
+When the solicitor finalises wording, edit the YAML files **and** bump
+this number in the same change so the version pointer stays accurate.
+"""
+from __future__ import annotations
+
+
+ACKNOWLEDGEMENT_VERSION: int = 1