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.toml

@@ -45,6 +45,7 @@ uuid = { version = "1", features = ["v4", "serde"] }
 
 [dev-dependencies]
 cookie = { version = "0.18", features = ["signed"] }
+tower = { version = "0.5", features = ["util"] }
 
 [profile.release]
 lto = "thin"