Add `created_at: Option<DateTime<Utc>>` through all layers so CLI users
can backdate entities at creation/update time. When omitted, falls back
to `Utc::now()`.
- Domain: add field to all New*/Update* structs with serde(default)
- Domain: timeline events use entity created_at instead of Utc::now()
- Repos: unify INSERT to explicit Rust-side created_at with unwrap_or_else
- Repos: add created_at to UPDATE dynamic query builders
- Routes: add field to submission structs and has_changes guards
- Clients: pass created_at through manual JSON client methods
- CLI: add --created-at flag with parse_created_at helper (RFC 3339 or YYYY-MM-DD)
The list-roasts API was returning nested objects with a "roast" field,
which was inconsistent with other list APIs in the codebase. Added
serde's #[serde(flatten)] attribute to make the response flat while
maintaining the internal struct composition.
