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/core/config.py

@@ -22,6 +22,18 @@ class Settings(BaseSettings):
     version: str = "0.0.0"
     app_env: str = Field(default="development", description="development | production")
 
+    # --- Instance owner / operator ---
+    # Email(s) of the instance owner(s) — the operator(s) who run this server.
+    # The matching account(s) get instance-admin rights (instance-wide settings;
+    # see /api/v1/admin). Comma-separated for several. Empty = no designated
+    # owner (the instance has no operator account). Derived at request time, so
+    # changing it takes effect immediately with no migration or DB state.
+    owner_email: str = ""
+
+    def owner_emails(self) -> frozenset[str]:
+        """Normalized (lowercased, trimmed) owner emails; empty if none set."""
+        return frozenset(e.strip().lower() for e in self.owner_email.split(",") if e.strip())
+
     # SQLAlchemy async URL, e.g. postgresql+asyncpg://user:pass@host:5432/db
     database_url: str = Field(
         default="postgresql+asyncpg://provenance:provenance@localhost:5432/provenance",