feat(search/lexical): populate SearchHit.indexed_at (fb-32)

JOIN documents.updated_at. stale defaults to false; App facade
post-processes against config threshold.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-search/Cargo.toml

@@ -25,6 +25,9 @@ serde_json      = { workspace = true }
 tracing         = { workspace = true }
 thiserror       = { workspace = true }
 anyhow          = { workspace = true }
+# p9-fb-32: parse documents.updated_at (RFC3339) into OffsetDateTime
+# for SearchHit.indexed_at.
+time            = { workspace = true }
 
 [dev-dependencies]
 tempfile        = { workspace = true }