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/auth.py

@@ -12,6 +12,7 @@ from app.config import settings
 from app.db import get_db
 from app.models import Invite, User
 from app.security import decrypt_optional
+from app.utils import canonical_email
 
 PASSWORD_MIN_LEN = 10
 
@@ -65,13 +66,29 @@ def has_write_scope(user: User) -> bool:
 
 def is_allowed(db: Session, email: str) -> bool:
     """May this email sign in? An approved Invite is the source of truth; the env
-    ALLOWED_EMAILS/ADMIN_EMAILS remain a bootstrap fallback (e.g. a fresh DB)."""
+    ALLOWED_EMAILS/ADMIN_EMAILS remain a bootstrap fallback (e.g. a fresh DB).
+
+    Matched on the CANONICAL form (utils.canonical_email), not the literal string. Gmail delivers
+    every dotted / `+tagged` spelling to one mailbox, so approving one spelling always meant approving
+    that mailbox — matching literally only produced the 2026-09-03 lockout, where an approved user's
+    Google sign-in became a silent second access request because OAuth returned the undotted primary.
+
+    One canonical form can match SEVERAL invite rows (prod carries approved pairs for one mailbox,
+    the oldest from 2026-06-16). An approved row among them grants access: the API cannot flip an
+    approved invite to denied (`_decide` only accepts a pending one), so approved-beside-denied means
+    the admin approved that mailbox under a different spelling, not that they revoked it. Revoking is
+    deleting the invite row or suspending the account — both still work, since a deleted row is no
+    longer here to match and suspension is enforced at sign-in.
+    """
     if not email:
         return False
+    canon = canonical_email(email)
+    statuses = (
+        db.execute(select(Invite.status).where(Invite.email_canonical == canon)).scalars().all()
+    )
+    if statuses:
+        return "approved" in statuses
     email = email.lower()
-    inv = db.execute(select(Invite).where(Invite.email == email)).scalar_one_or_none()
-    if inv is not None:
-        return inv.status == "approved"
     return email in settings.allowed_email_set or email in settings.admin_email_set
 
 
@@ -130,7 +147,10 @@ def purge_user(db: Session, user: User, background: BackgroundTasks) -> None:
     )
     # Raw delete so Postgres applies ON DELETE CASCADE / SET NULL on every dependent table.
     db.execute(delete(User).where(User.id == user_id))
-    db.execute(delete(Invite).where(Invite.email == email))
+    # Every canonically-equal invite goes, not just the literal spelling: is_allowed now matches on
+    # the canonical form, so leaving a sibling row behind would keep granting access to a mailbox
+    # whose account was just erased — and prod really does carry approved pairs for one mailbox.
+    db.execute(delete(Invite).where(Invite.email_canonical == canonical_email(email)))
     db.commit()
     if google_token:
         background.add_task(revoke_google_token, google_token)