chore: bump product to 0.6.0; multi-owner startup warning

Phase 3.5 ships → product minor bump under pre-1.0 rules (any surface
bump triggers minor). Schema is now 6 (0006_users_authz.sql); API
remains 1 (additive endpoints + new credential type, no breaking
shape changes). docs/versioning.md updated.

main.rs gets warn_on_multi_owner_install() which fires once after
bootstrap when more than one active owner exists — points the
operator at PATCH /admin/admins/{id} for cleanup. Soft-fail on DB
error (does not block startup).

The api-test schema assertion was updated to expect 6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/picloud/tests/api.rs

@@ -822,7 +822,7 @@ async fn version_includes_public_base_url(pool: PgPool) {
     let v: Value = r.json();
     assert!(v["public_base_url"].is_string());
     assert_eq!(v["api"], 1);
-    assert_eq!(v["schema"], 5);
+    assert_eq!(v["schema"], 6);
     assert_eq!(v["sdk"], "1.1");
 }