research: fix Draft 'Invalid Date' + stale-error leak in pipeline card

Two cosmetic bugs surfaced by the successful v0.8.3 pipeline run:

1. **'produced Invalid Date'** — research_outcomes.created_at was
   an OffsetDateTime serialized by time's default array format
   (`[y, ordinal, hh, mm, ss, ns, tz]`), which browser's
   `new Date(...)` can't parse. Add `#[serde(with =
   "time::serde::rfc3339")]` matching the pattern already in
   threads.rs / routine_runs.rs.

2. **Stale error text on pipeline card** — the runs stage's
   `latest_error` walked every run by `created_at DESC` and
   returned the first non-empty error, so a topic with an earlier
   failed run + a later completed run kept displaying the old
   error next to '1 completed'. Now the error only surfaces when
   the MOST RECENT run itself failed. Historical failures stay in
   the run count but don't leak their message.

crates/cm-db/src/repo/research_outcomes.rs

@@ -22,6 +22,11 @@ pub struct Outcome {
     pub version: i32,
     pub body_md: String,
     pub produced_by_run_id: Option<Uuid>,
+    // RFC3339 on the wire so `new Date(...)` in the browser parses it
+    // instead of choking on the `time` crate's default `[y, ordinal,
+    // ...]` array format (surfaced as "Invalid Date" in the Draft
+    // header).
+    #[serde(with = "time::serde::rfc3339")]
     pub created_at: OffsetDateTime,
 }