Multi-tenant: albums owned per photographer

- albums.owner_id (migration 0003, backfilled to the original user)
- owned::{album,photo,share} are the only admin data-access paths; another
  tenant's resources are indistinguishable from nonexistent (404)
- every admin handler threaded through ownership; each ALLOWED_EMAILS entry
  is now its own isolated workspace
- tenant-isolation integration test matrix (tests/tenancy.rs, env-gated on
  TEST_DATABASE_URL) driving the real router

Cargo.lock

@@ -2574,6 +2574,7 @@ dependencies = [
  "time",
  "tokio",
  "tokio-util",
+ "tower",
  "tower-http",
  "tracing",
  "tracing-subscriber",