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/models.py

@@ -246,6 +246,35 @@ class ReviewerVerdict(Base):
     model: Mapped[str | None] = mapped_column(String(64))
 
 
+class UserAcknowledgement(Base):
+    """Append-only record of a user actively accepting the sign-up
+    acknowledgement at ``/login``. See app/legal.py for the version
+    constant and app/locales/{en,it}.yaml::auth.ack for the canonical text.
+
+    One row per (user, version) acceptance. The value of this table is
+    evidentiary: 'user X accepted the version-N text on date D, in the
+    language they were shown'. A future wording revision bumps
+    ACKNOWLEDGEMENT_VERSION; existing users see the prompt again on next
+    sign-in and a new row is written at the new version."""
+    __tablename__ = "user_acknowledgements"
+    id: Mapped[int] = mapped_column(_PK, primary_key=True, autoincrement=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    version: Mapped[int] = mapped_column(SmallInteger, nullable=False)
+    # Language the user actually saw — what they accepted is keyed to it.
+    lang: Mapped[str] = mapped_column(String(8), nullable=False)
+    accepted_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=utcnow, nullable=False, index=True,
+    )
+
+    __table_args__ = (
+        Index("ix_user_acknowledgements_user_version", "user_id", "version"),
+    )
+
+
 # Portfolio / PortfolioSnapshot / Position removed in Phase G —
 # holdings live in the browser, the server stores only the anonymous
 # ticker universe + public market data.