Add an instance owner/operator role (env-declared via OWNER_EMAIL)

Provenance had no system-level owner: ownership was only per-tree
(TreeMembership), so a self-hosted instance had no operator account and no
instance-admin surface. This adds one, declared by environment per the project's
twelve-factor rule.

- OWNER_EMAIL (comma-separated): the account(s) named here are instance owners.
  Derived at request time — no DB column, no migration, can't drift from the env,
  survives DB resets. is_instance_owner()/InstanceOwner dependency in api/deps.py.
- Ownership requires a VERIFIED email (independent of REQUIRE_EMAIL_VERIFICATION).
  Registration is open, so without this an attacker could seize the role by
  registering the owner address first; verification ties it to inbox control.
- GET /api/v1/admin/instance (owner-only): operational status — version, env,
  user/tree counts, configured AI providers. Deliberately exposes no tree data
  or PII: instance ownership is an operator role, NOT a privacy-engine bypass.
- /users/me reports is_instance_owner; frontend gains an owner-only /admin page
  and a conditional sidebar link (server-enforced, not just client-hidden).

Found-and-fixed by an adversarial security review before merge: the
verified-email land-grab (above) and a frontend null-deref where the admin page
crashed on 401/5xx instead of failing closed.

Docs: .env.example + ARCHITECTURE (notes the not-a-privacy-bypass boundary and
the verified-email requirement). Tests: owner matching, the land-grab guard,
/users/me, and owner-only /admin. Suite 96 passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/api/deps.py

@@ -54,6 +54,36 @@ async def get_current_user_or_none(request: Request, session: SessionDep) -> Use
 CurrentUserOrNone = Annotated[User | None, Depends(get_current_user_or_none)]
 
 
+def is_instance_owner(user: User) -> bool:
+    """Whether this account is an instance owner/operator — i.e. its email is
+    named in OWNER_EMAIL *and* that email has been verified. Instance ownership
+    is an operational/config role; it does NOT bypass the privacy engine or grant
+    access to others' tree data.
+
+    The verified-email requirement is load-bearing: registration is open and (by
+    default) doesn't require verification, so without it an attacker could claim
+    the owner email by registering it before the operator does — a land-grab to
+    the highest role with no proof of inbox control. Requiring verification ties
+    ownership to actual control of the named inbox regardless of the global
+    REQUIRE_EMAIL_VERIFICATION setting. (Self-hosts without SMTP can verify via
+    the link the console mailer prints to the operator-controlled logs.)"""
+    owners = get_settings().owner_emails()
+    return (
+        bool(owners)
+        and user.email_verified_at is not None
+        and user.email.strip().lower() in owners
+    )
+
+
+async def require_instance_owner(current: CurrentUser) -> User:
+    if not is_instance_owner(current):
+        raise HTTPException(status.HTTP_403_FORBIDDEN, "instance owner only")
+    return current
+
+
+InstanceOwner = Annotated[User, Depends(require_instance_owner)]
+
+
 def get_mailer() -> Mailer:
     settings = get_settings()
     if settings.mailer == "smtp" and settings.smtp_host: