fix(auth): match identity on a canonical email, so one Gmail mailbox is one account (E9.5 S3)

Gmail delivers x.y@, xy@ and xy+tag@ to one inbox; the invite gate only lowercased.
On 2026-09-03 that turned an approved user's Google sign-in into a silent SECOND
access request -- OAuth returns the account's stored primary, which need not be the
spelling they typed -- and left prod with two user rows for one mailbox (ids 13, 14).
The +tag case fails every time: a Google account cannot contain +, so OAuth can only
ever answer with the base address.

Mail still goes to the address AS TYPED; only identity matching uses the canonical
form. utils.canonical_email is the single definition, and a model validator derives
users.email_canonical / invites.email_canonical on every write, so no call site can
forget it. Scope is narrow on purpose: dots are significant on most providers and the
Gmail rule does not hold for Workspace custom domains.

Swept every sibling of the same principle, not just the reported one: is_allowed,
upsert_pending_invite, the Google-callback adoption, _register_account (so no new
duplicate pair can be created), _activate_user_by_email, admin _accounts_for (an
invite backing a live user under another spelling used to look deletable) and
add_invite, plus purge_user, which now erases every canonically-equal invite instead
of leaving one behind that still grants access.

Migration 0079 backfills both columns and indexes them NON-uniquely: prod already
carries canonical duplicates (approved invite pairs since 2026-06-16), and merging
accounts is a judgement call about people's data, not something a migration does on
its own. A test pins the backfill SQL against the Python helper.

backend/app/models.py

@@ -17,9 +17,10 @@ from sqlalchemy import (
     func,
 )
 from sqlalchemy.dialects.postgresql import JSONB, TSVECTOR
-from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.orm import Mapped, mapped_column, relationship, validates
 
 from app.db import Base
+from app.utils import canonical_email
 
 
 class TimestampMixin:
@@ -29,6 +30,21 @@ class TimestampMixin:
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
 
 
+class CanonicalEmailMixin:
+    """Keeps `email_canonical` derived from `email` on every write.
+
+    The invariant — "identity is matched on the canonical form, mail is sent to the typed one" —
+    lives HERE, in code, instead of in a comment every write site has to remember. That distinction
+    is what the 2026-09-03 lockout was made of: one gate lowercased, nothing else did, and an
+    approved user's Google sign-in silently became a second access request.
+    """
+
+    @validates("email")
+    def _derive_email_canonical(self, _key: str, value: str | None) -> str | None:
+        self.email_canonical = canonical_email(value)
+        return value
+
+
 class UpdatedAtMixin:
     """Adds an `updated_at` that server-defaults on insert and bumps on every UPDATE."""
 
@@ -37,7 +53,7 @@ class UpdatedAtMixin:
     )
 
 
-class User(Base, TimestampMixin):
+class User(Base, TimestampMixin, CanonicalEmailMixin):
     __tablename__ = "users"
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -45,6 +61,11 @@ class User(Base, TimestampMixin):
     # multiple NULLs under a unique index). Set on Google sign-in / in-app linking.
     google_sub: Mapped[str | None] = mapped_column(String(64), unique=True, index=True)
     email: Mapped[str] = mapped_column(String(320), unique=True, index=True)
+    # The same mailbox, in the form used to MATCH identities (utils.canonical_email) — Gmail dots and
+    # `+tags` folded away. Kept by the validator below so no write site can forget it. Deliberately
+    # NOT unique: prod already carries two rows for one mailbox (a password registration and the
+    # Google sign-in that failed to find it), and a migration must not merge accounts on its own.
+    email_canonical: Mapped[str] = mapped_column(String(320), index=True, server_default="")
     # argon2id hash for email+password login; NULL for Google-only / demo accounts.
     password_hash: Mapped[str | None] = mapped_column(String(255))
     # When SEC-10 dropped a preset password on Google adoption (see the single caller of
@@ -121,7 +142,7 @@ class AuthToken(Base, TimestampMixin):
     used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
 
 
-class Invite(Base):
+class Invite(Base, CanonicalEmailMixin):
     """Access-request / whitelist row. Source of truth for who may sign in; the env
     ALLOWED_EMAILS/ADMIN_EMAILS act only as a bootstrap fallback (see auth.is_allowed)."""
 
@@ -129,6 +150,9 @@ class Invite(Base):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     email: Mapped[str] = mapped_column(String(320), unique=True, index=True)
+    # See User.email_canonical. Not unique here either: prod has approved invites for both spellings
+    # of the same mailbox, dating back to 2026-06-16.
+    email_canonical: Mapped[str] = mapped_column(String(320), index=True, server_default="")
     # pending | approved | denied
     status: Mapped[str] = mapped_column(String(16), default="pending", server_default="pending")
     note: Mapped[str | None] = mapped_column(Text)