Per-channel effective capabilities + provider manifests

Newest first, keyset-paginated via `next_cursor`. Narrow with `statuses`, a scheduled-time range, `campaign`, `label`, or `channel` (a channel id or a provider name like `linkedin`) — so "my scheduled LinkedIn posts" is one call, not a range pulled down and filtered locally. Rows are excerpts and carry no fit verdict unless you ask: `include=ready` adds `ready` per row — absent means "not checked", which is not the same as `false` — and caps the page at 25, because each judged row then costs a full post read. Read one post in full with `GET /posts/{id}`.