feat(app): staleness module + post-process search hits (fb-32)

compute_stale: strict > boundary, threshold=0 disables, future
timestamps treated as fresh (clock skew safety). App::search
re-stamps on cache hit so config threshold changes take effect
without flushing the cache.

Also unblocks the workspace build by plugging placeholder
indexed_at/stale into the two AnswerCitation construction
sites in kebab-rag/pipeline.rs (the score-gate refusal path
forwards from SearchHit; the LLM-citation path uses
UNIX_EPOCH/false until Task 7 wires the real values through
pack_context).

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

crates/kebab-app/tests/search_stale_integration.rs

@@ -0,0 +1,87 @@
+//! p9-fb-32: `App::search` end-to-end staleness wiring.
+//!
+//! `compute_stale` itself is unit-tested in `kebab_app::staleness`; this
+//! file proves the post-process actually fires through the full
+//! retriever stack and that the cache-hit re-stamp respects the
+//! configured threshold.
+//!
+//! All three tests run lexical-only (no AVX, no fastembed download).
+
+mod common;
+
+use common::TestEnv;
+
+fn lexical_query_owner() -> kebab_core::SearchQuery {
+    common::lexical_query("ownership")
+}
+
+/// Fresh ingest at default 30-day threshold → no hit can be stale.
+/// `documents.updated_at` is stamped at ingest time (now), so the
+/// distance to `now_utc()` is sub-second.
+#[test]
+fn fresh_doc_is_not_stale_with_default_threshold() {
+    let env = TestEnv::lexical_only();
+    kebab_app::ingest_with_config(env.config.clone(), env.scope(), true).unwrap();
+
+    let app = kebab_app::App::open_with_config(env.config.clone()).unwrap();
+    let hits = app.search(lexical_query_owner()).unwrap();
+    assert!(!hits.is_empty(), "expected ≥1 hit for 'ownership'");
+    assert!(
+        hits.iter().all(|h| !h.stale),
+        "freshly-ingested doc must not be stale at default 30d threshold: {:?}",
+        hits.iter().map(|h| (h.doc_path.0.clone(), h.stale)).collect::<Vec<_>>()
+    );
+}
+
+/// `stale_threshold_days = 0` disables the feature even for very old
+/// `documents.updated_at`. Backdate the row to a year ago, expect
+/// `stale: false` on every hit.
+#[test]
+fn threshold_zero_disables_staleness() {
+    let mut env = TestEnv::lexical_only();
+    env.config.search.stale_threshold_days = 0;
+
+    kebab_app::ingest_with_config(env.config.clone(), env.scope(), true).unwrap();
+    common::backdate_document_updated_at(&env, "intro.md", 365);
+
+    let app = kebab_app::App::open_with_config(env.config.clone()).unwrap();
+    let hits = app.search(lexical_query_owner()).unwrap();
+    assert!(!hits.is_empty(), "expected ≥1 hit");
+    assert!(
+        hits.iter().all(|h| !h.stale),
+        "threshold=0 disables staleness even for year-old docs: {:?}",
+        hits.iter().map(|h| (h.doc_path.0.clone(), h.stale)).collect::<Vec<_>>()
+    );
+}
+
+/// At a 30-day threshold, a 60-day-old `documents.updated_at` must
+/// surface as stale on the matching hit. (Other hits — fresh fixtures
+/// not backdated — stay fresh, so we use `any` not `all`.)
+#[test]
+fn old_doc_marked_stale() {
+    let mut env = TestEnv::lexical_only();
+    env.config.search.stale_threshold_days = 30;
+
+    kebab_app::ingest_with_config(env.config.clone(), env.scope(), true).unwrap();
+    common::backdate_document_updated_at(&env, "intro.md", 60);
+
+    let app = kebab_app::App::open_with_config(env.config.clone()).unwrap();
+    let hits = app.search(lexical_query_owner()).unwrap();
+    assert!(!hits.is_empty(), "expected ≥1 hit");
+    let intro_hits: Vec<&kebab_core::SearchHit> = hits
+        .iter()
+        .filter(|h| h.doc_path.0.ends_with("intro.md"))
+        .collect();
+    assert!(
+        !intro_hits.is_empty(),
+        "expected ≥1 hit on intro.md (the backdated doc)"
+    );
+    assert!(
+        intro_hits.iter().all(|h| h.stale),
+        "60-day-old intro.md must be stale at 30d threshold: {:?}",
+        intro_hits
+            .iter()
+            .map(|h| (h.doc_path.0.clone(), h.stale))
+            .collect::<Vec<_>>()
+    );
+}