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>

deploy/.env.example

@@ -4,6 +4,18 @@
 # --- Core ---
 APP_ENV=development
 
+# Instance owner / operator. The account(s) whose email is named here get
+# instance-admin rights (the owner-only /admin surface, instance-wide settings).
+# Comma-separated for several owners. Leave empty for an instance with no
+# designated operator. Derived at request time — no migration, takes effect on
+# restart. Set this to YOUR account email on a real deployment.
+#
+# The named account must have a VERIFIED email to be recognized as owner — this
+# stops someone from claiming the owner address by registering it before you do.
+# Register this email and verify it (via SMTP, or the link the console mailer
+# prints to the backend logs) — ideally before exposing registration publicly.
+OWNER_EMAIL=
+
 # --- Images (pulled from git.jpaul.io; CI pushes to the LAN registry) ---
 # test-main = current main build; or pin a semver / test-sha-<sha> for rollback.
 IMAGE_TAG=test-main