feat(app): log retention — keep_recent_runs + retention_days (Enhancement 4)

LoggingCfg gains two fields with serde defaults: keep_recent_runs
(default 100, top-N file retention) and retention_days (default 30,
time-based retention for both ndjson files and the SQLite mirror).

IngestLogWriter::open now runs cleanup_old_logs before creating a new
ingest-*.ndjson — delete iff (idx >= keep_recent) OR (modified <=
cutoff). ingest_with_config_opts also calls
SqliteStore::prune_pdf_ocr_events(retention_days) at ingest start so
the SQLite mirror tracks the same retention window.

Backward compat (AC-9): both new fields use #[serde(default = ...)],
so a pre-v0.20.x config with only [logging] ingest_log_enabled +
ingest_log_dir parses unchanged. kebab init writes the new defaults
automatically via Config::default() -> toml::to_string_pretty (AC-12).

docs/SMOKE.md config example synced.

Closure r1 F5: explicit OR-on-stale comment inside cleanup_old_logs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/kebab-app/tests/ingest_log_smoke.rs

@@ -28,6 +28,7 @@ fn minimal_config(workspace: &std::path::Path, log_dir: &std::path::Path) -> Con
     cfg.logging = LoggingCfg {
         ingest_log_enabled: true,
         ingest_log_dir: log_dir.to_path_buf(),
+        ..Default::default()
     };
     cfg
 }
@@ -138,6 +139,7 @@ fn ingest_log_disabled_emits_no_file() {
     cfg.logging = LoggingCfg {
         ingest_log_enabled: false,
         ingest_log_dir: log_dir.clone(),
+        ..Default::default()
     };
 
     let scope = SourceScope {

crates/kebab-app/tests/pdf_ocr_events_insert_smoke.rs

@@ -58,6 +58,7 @@ async fn ingest_dual_write_doc_id_matches_ndjson() {
         env.config.logging = LoggingCfg {
             ingest_log_enabled: true,
             ingest_log_dir: log_dir.clone(),
+            ..Default::default()
         };
 
         // Copy scanned PDF into workspace