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/utils.py

@@ -22,6 +22,39 @@ def valid_email(value: str | None) -> bool:
     return bool(value and _EMAIL_RE.match(value))
 
 
+# Gmail (and its googlemail.com alias) delivers to one mailbox regardless of dots in the local part
+# and of any `+tag`; nobody else is guaranteed to. So the canonical form below strips those for
+# Gmail only, and merely lowercases everything else.
+_GMAIL_DOMAINS = {"gmail.com", "googlemail.com"}
+
+
+def canonical_email(value: str | None) -> str:
+    """The form used to decide whether two addresses are the SAME MAILBOX. Never used for sending —
+    mail always goes to the address as the user typed it.
+
+    Why this exists (observed on prod, 2026-09-03): someone registered as `npeter83.temu@gmail.com`
+    and was approved, but Google's OAuth `email` claim came back as the account's stored primary,
+    `npeter83temu@gmail.com`. The invite gate only lowercased, found no approved row for that literal
+    string, and turned an approved user's sign-in into a silent SECOND access request — plus a second
+    User row. The `+tag` case is worse and fails every time: a Google account cannot contain `+`, so
+    OAuth can only ever return the base address, meaning anyone who registers with
+    `user+siftlode@gmail.com` (a common throwaway habit) is guaranteed to be denied after approval.
+
+    Scope is deliberately narrow. The dot rule does NOT hold for Google Workspace custom domains
+    unless the admin turned it on, and dots are significant on most providers, so anything outside
+    gmail.com/googlemail.com is only lowercased. Approving one Gmail spelling always did mean
+    approving that mailbox, so collapsing them is the honest reading of an invite, not a widening
+    of it.
+    """
+    email = (value or "").strip().lower()
+    local, sep, domain = email.partition("@")
+    if not sep or domain not in _GMAIL_DOMAINS:
+        return email
+    local = local.partition("+")[0].replace(".", "")
+    # Normalise the alias domain too, so googlemail and gmail spellings meet.
+    return f"{local}@gmail.com" if local else email
+
+
 def now_utc() -> datetime:
     """Current time as a timezone-aware UTC datetime."""
     return datetime.now(timezone.utc)