perf(db): persist `featured` as a spec column

[milkway]

Audit finding (perf). The Apps list (routes/admin/specs.rs:297) and the public landing (routes/landing.rs:201) call list_all/effective_specs, which SELECT config_json and JSON-parse every spec per render, just to read is_featured() and a few fields. config_json is the largest column.

Fix: persist featured (like state/kind) as its own column and select it in the lean query; short-TTL cache for effective_specs on the landing.